Alloy Selection for Sulfuric Acid: Alloy 20 vs 28 vs C-276 (58 chars)

By Shanghai Hangbo Alloy Group Co., Ltd. - China Supplier | ISO 9001:2015 | Est. 2012

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Quick Answer: Sulfuric Acid Alloy Selection

Pick Alloy 20 for dilute sulfuric acid up to moderate temperature, Alloy 28 where that dilute acid runs hot, and C-276 where chlorides, fluorides or reducing conditions dominate. Carbon steel stays correct for 98-99% acid at ambient temperature, and 904L or a 6Mo grade is the economic middle.

Key Takeaways

• Dilute acid is the aggressive region, concentrated acid is not. Below roughly 70% H2SO4 the cathodic reaction is proton reduction and no protective film survives on steel; at 96-99% the acid itself is the oxidiser and passivates carbon steel at ambient temperature.
• Chlorides and fluorides decide the grade, not the acid strength. They depress the pitting resistance of every austenitic option and are the usual reason an Alloy 20 part fails where Alloy 20 "should" have worked.
• Alloy 20 (N08020) is the default dilute-acid grade. Copper plus molybdenum gives it acid resistance that 904L and 316L cannot match, at roughly half the cost of C-276.
• Alloy 28 (N08028) is the hot dilute answer. Its 26-28% chromium holds up where dilute acid temperature pushes past the Alloy 20 envelope.
• Anodic protection changes the answer. A potentiostat can hold 316L passive in dilute acid service, replacing an alloy upgrade with an instrument loop.
• Velocity and weld condition set the real service life. Acid inlet impingement, weld decay and under-deposit pitting account for most failures in acid circuits, and none of them is a grade-selection error.

Why Dilute Sulfuric Acid Is the Aggressive Region and Why 98-99% Acid Runs in Carbon Steel

Dilute sulfuric acid attacks steel because the only cathodic reaction available is proton reduction, and no protective film can form under it. Concentrated acid behaves differently: at 96-99% the water activity is so low that the acid itself acts as a strong oxidiser, and carbon steel passivates under a sulphate film. That single electrochemical fact explains the apparent paradox at the top of every acid plant.

In dilute acid, hydrogen evolution sets the corrosion rate. Steel in 5-20% H2SO4 at ambient temperature corrodes at rates measured in millimetres per year, and aeration makes it worse rather than better because oxygen reduction adds a second cathodic reaction. Austenitic stainless steels do better only because they can passivate, and they can passivate only if the acid is oxidising enough or if an oxidising species is present. This is why 316L survives dilute acid in some circuits and dissolves in others with no obvious change in analysis: the difference is dissolved oxygen, ferric ion, or an oxidiser such as nitric acid or hydrogen peroxide in the stream.

Above roughly 90% concentration the picture inverts. The acid has very little free water, the sulphate film on carbon steel becomes stable, and carbon steel storage tanks, rail cars and 98% acid piping have been standard practice for a century. The engineering limits are temperature and velocity, not concentration. Ambient temperature is comfortable, moderate warming is tolerable in many plants, and hot concentrated acid is a different problem again, handled by high-silicon iron, Alloy 20 in specific conditions, or lined equipment.

The dangerous zone is the boundary. A water line, a wash-water connection, a dilute acid return or a leaking steam coil that puts water into a 98% acid tank creates a local dilute pocket that attacks the steel shell at a rate that can perforate a plate in weeks. The same failure appears at the interface in a tank that stratifies after a water addition, and at the bottom of a tank where dilute acid collects under concentrated acid. Every concentrated-acid specification should therefore include a mixing and water-ingress rule, not only a material rule.

Concentration band	Electrochemical character	Typical alloy practice	Governing concern
Below 20% H2SO4	Strongly reducing at the steel surface, proton reduction	Alloy 20, Alloy 28, 904L, 6Mo; 316L only when oxidised	Velocity, chlorides, aeration
20-70% H2SO4	Reducing to mixed, strongly temperature dependent	Alloy 20 to moderate temperature; Alloy 28 or C-276 when hot	Temperature and chloride level
70-90% H2SO4	Transition, oxidising character develops	904L, 6Mo, Alloy 20 at ambient; carbon steel only in narrow conditions	Local dilution, velocity
96-99% H2SO4	Oxidising, low water activity, sulphate film stable	Carbon steel at ambient, low velocity; 316L or 904L where velocity or colour must be controlled	Water ingress, velocity, temperature
Oleum and 100%+	Strongly oxidising, SO3 present	Carbon steel with strict exclusion of water	Water ingress, sealing, venting

Table note: concentration bands describe typical plant practice, not standard limits. Alloy direction follows immersion and field experience reported to ASTM G31 and ASTM G28 test frameworks; the acceptability of any grade must be confirmed against the actual stream analysis, temperature and velocity.

Aeration, Velocity and the Oxidising-Reducing Balance

Dissolved oxygen is the cheapest corrosion inhibitor in sulfuric acid service and the most unreliable one. Where oxygen or another oxidiser is reliably present, stainless steels hold their passive film and 904L or a 6Mo grade can serve in dilute acid that would destroy them in a deaerated stream. Where oxygen is excluded, the same alloys can swing into active dissolution and the corrosion rate jumps by orders of magnitude.

This makes the oxidising-reducing balance a design input, not a footnote. A closed tank blanketed with nitrogen, a deaerated feed, a stream that has consumed all its oxygen in an upstream reaction and a circuit with active sulphide contamination all push the alloy toward the reducing end, and the correct response is a higher molybdenum grade or an added oxidiser. The converse is also true: an air sparge, a nitrate addition or a ferric ion content can rescue a cheaper alloy, and plants frequently manage dilute acid service that way rather than paying for C-276.

The trap is variability. An oxidiser that is present during normal operation may be absent during a shutdown, a wash, a feed interruption or an upset, and the alloy that was passive for two years corrodes in the third. Any selection based on oxidiser control should state the minimum oxidiser concentration, the measurement method and what happens when it is lost. A stream that cannot guarantee the oxidiser should be selected as a reducing stream.

Velocity is the second variable that moves a stream across the passive-active boundary. A passive film repairs itself from the bulk liquid, and flow replenishes the ions that repair needs, so moderate velocity is beneficial in clean acid. Once the acid carries solids, or once the wall shear stress is high enough to detach the film mechanically, velocity becomes destructive. The transition is sharper in acid than in neutral service because the film is thinner and less able to survive disturbance.

Temperature acts in the same direction on all of these variables. It raises the corrosion rate of the active state, it lowers the stability of the passive film, and it shortens the incubation time for pitting in chloride-bearing acid. A grade that is comfortable at 40 °C in a given concentration is often unacceptable at 80 °C, and the reclassification usually happens over a narrow band rather than gradually. This is why acid service grades should always be selected against a specific temperature and a specific chloride level, not against a generic "sulfuric acid" label on a data sheet.

Chlorides and Fluorides: The Variables That Actually Decide the Grade

In real acid circuits, chloride and fluoride content decides the grade far more often than acid concentration does. Chloride breaks down the passive film locally and starts pitting at a potential that falls as temperature rises; fluoride attacks the film uniformly and is especially damaging in mixed acid streams such as those from pickling, phosphoric acid digestion and smelter gas cleaning.

The mechanism is straightforward. Chloride adsorbs on the passive film, displaces oxygen at defect sites and, above a critical potential, causes local breakdown. Once a pit is stable the interior becomes an acidic, chloride-rich cell that keeps growing even if the bulk conditions recover. Practical experience in acid circuits places the risk threshold far lower than the number a materials engineer would assume from neutral water service: a few hundred parts per million of chloride at 60-80 °C is enough to eliminate 316L and 904L from consideration in many streams, and Alloy 20 has a finite chloride ceiling too. Alloy 28 and the 6Mo grades raise the ceiling, and C-276 raises it much further, but none of them is unlimited.

Fluoride works differently and is often overlooked because it does not produce a visible pit. It complexes the metal ions needed to rebuild the film, so it thins and dissolves the film generally rather than penetrating it locally. In hydrofluoric acid contamination, in fluoride-bearing pickling liquor and in phosphate rock digestion, fluoride is frequently the variable that kills an alloy which was selected purely on sulfuric acid concentration. Where both halides are present, as in wet-process phosphoric acid and in many smelter acid streams, the selection should be based on the combined analysis rather than on chloride alone.

There is a third halide-adjacent variable worth naming: oxidiser interaction. Chloride pitting requires a potential above the breakdown potential, and an oxidiser raises the potential while a reducing condition lowers it. So the same chloride level can be tolerable in a deaerated stream and unacceptable in an aerated one, and a chlorinated biocide addition can convert a comfortable duty into a pitting failure within one campaign. Historical records for a circuit should be read for chlorination events, not only for average chloride.

The composition table below sets out the grades discussed in this guide. Values are standard composition ranges from the product specifications named; a purchase order should always be written against the current edition of the applicable specification and confirmed on the mill certificate.

Grade	UNS	Standard	Ni %	Cr %	Mo %	Cu %	Other (range)
Alloy 20	N08020	ASTM B463 / B473	32.0-38.0	19.0-21.0	2.0-3.0	3.0-4.0	Nb+Ta 8xC-1.0, C 0.07 max, Fe balance
Alloy 28	N08028	ASTM B668	30.0-34.0	26.0-28.0	3.0-4.0	0.6-1.4	C 0.02 max, Mn 2.0 max, Fe balance
904L	N08904	ASTM B625 / B677	23.0-28.0	19.0-23.0	4.0-5.0	1.0-2.0	C 0.020 max, N 0.10 max, Fe balance
6Mo (254 SMO type)	S31254	ASTM A240	17.5-18.5	19.5-20.5	6.0-6.5	0.5-1.0	N 0.18-0.22, C 0.020 max
6Mo (AL-6XN type)	N08367	ASTM B688 / A240	23.5-25.5	20.0-22.0	6.0-7.0	0.75 max	N 0.18-0.25, C 0.030 max
Alloy 31	N08031	EN 1.4562 (W.Nr)	30.0-32.0	26.0-28.0	6.0-7.0	1.0 max	N 0.15-0.25, C 0.015 max
Alloy 33	R20033	EN 1.4591 (W.Nr)	30.0-33.0	31.0-35.0	0.5-2.0	0.3-1.2	N 0.35-0.60, C 0.015 max
C-276	N10276	ASTM B575 / B622	Balance	14.5-16.5	15.0-17.0	-	W 3.0-4.5, Fe 4.0-7.0, C 0.01 max
316L	S31603	ASTM A240	10.0-14.0	16.0-18.0	2.0-3.0	-	C 0.030 max, Fe balance

Table note: ranges as published in the listed product specifications (ASTM B463, B668, B575, B625, B688, A240; EN 1.4562 and EN 1.4591 for the European grades). Trace elements and residuals are omitted for readability; the specification governs. Alloy 31 and Alloy 33 are procured to European designations, so the ASTM or GB/T equivalent must be agreed in writing on the purchase order.

Alloy 20: Copper Plus Molybdenum and Why It Still Sets the Dilute-Acid Benchmark

Alloy 20 (UNS N08020) is an austenitic nickel-iron-chromium alloy with deliberately added copper and molybdenum, and it remains the reference grade for sulfuric acid at moderate temperature. The copper addition is the differentiator: it shifts the alloy's corrosion potential in dilute acid such that the passivation range widens and the active dissolution rate falls, and molybdenum suppresses attack in the reducing regions that remain. Neither element does the job alone.

That combination is why Alloy 20 handles streams that defeat 904L, and why it does so at a fraction of the cost of a Ni-Mo-Cr grade. It is also why Alloy 20 is specified across a very wide set of acid-adjacent services: sulfuric acid pickling, phosphate acidulation, acid coolers, spent acid recovery, alkylation unit piping, tank farm transfer lines and, in the original development, acid-resistant castings for the chemical industry. The grade is stabilised with niobium, which ties up carbon as carbide and reduces the risk of sensitisation during welding, an important property in fabricated vessels and pipe spools.

The limits matter as much as the capability. Alloy 20 is not a chloride alloy. Its molybdenum content is modest by super-austenitic standards, its chromium is around 20%, and its pitting resistance equivalent is correspondingly limited, so chloride-bearing acid at elevated temperature is outside its comfortable envelope. It is also not the right answer for strongly oxidising acid, where a lower-alloy stainless may be perfectly adequate and cheaper, nor for hot concentrated acid, where 904L, 6Mo or a nickel-based grade may be preferred depending on the specific stream.

Procurement of Alloy 20 needs care because the grade exists in several product forms under different specifications, and "Alloy 20" alone is not a specification. Plate, sheet and strip are covered by ASTM B463, bar and rod by ASTM B473, seamless pipe and tube by ASTM B729, welded pipe by ASTM B464 and fittings by ASTM B366. A purchase order that names only the trade name leaves the mill free to supply a form and condition that may not match the design, and it is a common cause of documentation disputes at goods receipt.

Mechanical properties are secondary in acid service but still set the design allowables. The table below gives typical room-temperature values for solution-annealed material; the specification minima for the grade you actually buy must be read from the applicable standard and from the mill certificate.

Grade	Standard	UTS (MPa, typical)	0.2% yield (MPa, typical)	Elongation (%, typical)	Hardness (typical)
Alloy 20	ASTM B463 / B473	620	240	30-45	140-180 HB
Alloy 28	ASTM B668	600	250	40	150-190 HB
904L	ASTM B625 / B677	620	260	40	150-200 HB
6Mo (S31254)	ASTM A240	690	310	35	180-220 HB
Alloy 31	EN 1.4562	750	350	40	180-220 HB
Alloy 33	EN 1.4591	750	380	40	190-230 HB
C-276	ASTM B575 / B622	790	355	40	185-210 HB
316L	ASTM A240	485	170	40	130-180 HB

Table note: all values are typical, solution-annealed, room temperature, and are not standard minima. Tensile testing is performed to ASTM E8/E8M; hardness to ASTM E18 or E10 as applicable. Specification minima and any elevated-temperature properties must be taken from the edition of the product standard cited on the purchase order.

Alloy 28 for Hot Dilute Acid: Where Alloy 20 Runs Out of Temperature

Alloy 28 (UNS N08028) is the grade to reach for when the acid is dilute and hot. Its chromium content sits at 26-28%, roughly a third higher than Alloy 20, and that additional chromium is what holds the passive film together as temperature rises. The grade keeps enough nickel to remain fully austenitic and weldable, and enough molybdenum and copper to handle the reducing side of the acid.

The practical difference between the two grades is temperature, not concentration. In the same dilute acid, Alloy 20 gives good service up to a moderate ceiling and then begins to lose the film; Alloy 28 extends that ceiling meaningfully and covers dilute acid that would put Alloy 20 into active corrosion. This is why Alloy 28 appears in acid coolers and heat exchangers, in dilute acid return lines, in sulfuric acid dilution loops, in pickling line equipment handling mixed nitric-hydrofluoric acid, and in phosphoric acid duties where the temperature is high and the chloride level is not extreme.

Its weakness, shared with every high-chromium austenitic, is chloride pitting under deposits and in crevices. A high chromium content raises the pitting resistance but does not make the alloy immune, and Alloy 28 is more sensitive to crevice geometry than C-276 because it lacks the very high molybdenum level that stabilises the film in a chloride-rich pit. Any design that leaves a gasket crevice, a weld lap or a sludge deposit in Alloy 28 equipment should be reviewed, because that is where the first penetration will appear.

Alloy 28 is supplied principally as seamless tube and pipe under ASTM B668, and that product form drives most of its use: heat exchanger tubing, cooler bundles and small-diameter acid lines. Plate and bar are available but the grade's centre of gravity is tubular, which is relevant at the procurement stage because tube mill availability and tube-to-tubesheet welding qualification both need to be confirmed early. The European designation is EN 1.4563, widely referenced in European acid plant specifications.

Service condition	Alloy 20	Alloy 28	904L / 6Mo	C-276
Dilute H2SO4, ambient to moderate temperature	Preferred	Suitable	Suitable when oxidised	Over-specified
Dilute H2SO4, hot	Limited	Preferred	Limited	Suitable
Dilute H2SO4 with chloride above a few hundred ppm	Not recommended	Conditional	Conditional	Preferred
Mixed acid with fluoride	Conditional	Suitable	Not recommended	Preferred
Hot concentrated H2SO4	Not recommended	Not recommended	Conditional	Conditional
Oxidising acid with nitrate or peroxide	Suitable	Suitable	Preferred	Suitable but costly

Table note: the table expresses typical selection practice as service envelopes, not standard limits. Each cell assumes clean, weldable material in the solution-annealed condition and must be confirmed against the actual analysis, temperature, velocity and oxidiser level; corrosion testing to ASTM G31 with the real liquor is the correct confirmation step for any borderline duty.

904L and the 6Mo Super-Austenitics: The Economic Middle

Between the standard austenitics and the nickel-based grades sits a group that covers a large share of real acid duties at a defensible price: 904L (N08904) and the 6% molybdenum super-austenitics, principally the 254 SMO type (S31254) and the AL-6XN type (N08367). They are not interchangeable, and confusing them is one of the more expensive mistakes in acid-service procurement.

904L is a high-nickel, high-molybdenum austenitic stainless with copper, developed for sulfuric acid service and widely used in acid plant piping, cooler tubing, scrubber internals and phosphoric acid equipment. It is fully austenitic, tough, readily welded and available in every product form, which makes it the practical default when a 316L part has failed but the duty does not justify a nickel alloy. Its 4-5% molybdenum and 1-2% copper give it real sulfuric acid capability up to moderate temperature, and its copper content is why it performs better in dilute acid than its chromium and molybdenum alone would suggest.

The 6Mo grades move the emphasis toward chlorides. With 6-7% molybdenum and 0.18-0.25% nitrogen, they achieve a pitting resistance equivalent number around 40-45 and tolerate chloride levels and temperatures that defeat 904L, at the cost of higher hardness, lower weldability margin and a price close to Alloy 20. They are the correct answer for chloride-bearing acid where the temperature is moderate and the duty is oxidising, and they are frequently the cheapest grade that will actually survive a seawater-cooled acid cooler.

Grade	UNS	PREn (typical, calculated)	Critical pitting temperature (typical)	Relative cost band	Best fit
316L	S31603	24-26	Below 20 °C in 6% FeCl3	Low	Clean, oxidised, low-chloride acid
904L	N08904	34-36	25-40 °C	Medium	Sulfuric acid, moderate temperature
6Mo (S31254)	S31254	42-44	50-60 °C	Medium-high	Chloride-bearing acid, oxidising
6Mo (N08367)	N08367	42-45	50-60 °C	Medium-high	Chloride-bearing acid, welded structures
Alloy 28	N08028	37-39	40-50 °C	Medium-high	Hot dilute acid
Alloy 31	N08031	48-52	65-80 °C	High	Chloride acid with oxidising duty
C-276	N10276	65-70	Above 85 °C	Very high	Chloride, fluoride, reducing acid

Table note: PREn values are typical calculations from the composition mid-point using the chromium + 3.3 x molybdenum + 16 x nitrogen convention and are indicative only; they are not a specification property. Critical pitting temperatures are typical values from ASTM G48 Practice C style testing in 6% ferric chloride and must not be treated as guaranteed minima or as design limits for a specific liquor.

The economic argument for this group is simple arithmetic. Where 904L or a 6Mo grade will hold, specifying C-276 multiplies material cost by roughly two to three times for capability that is not being used. The opposite error is equally common: forcing 316L or 904L into a chloride-bearing duty because the acid concentration "looks low", and then replacing the part annually. The correct test is not concentration but the combination of chloride, temperature, oxidiser and velocity, and the moment two of those four move in the aggressive direction, the answer usually moves up one grade.

C-276, Alloy 31 and Alloy 33 for Chloride-Bearing and Reducing Duties

When the acid carries chlorides, fluorides or a genuinely reducing character, the selection moves to the Ni-Cr-Mo family, and C-276 (N10276) is its reference member. Its 15-17% molybdenum with tungsten gives it exceptional resistance to reducing acid and to chloride pitting and crevice attack, and its low carbon plus controlled silicon make it resistant to grain-boundary precipitation in the weld heat-affected zone. For a sulfuric acid stream that also contains chlorides, C-276 is usually the grade that stops the failure.

C-276 is not a universal acid alloy, and specifying it everywhere is both wasteful and occasionally wrong. In strongly oxidising acid it is attacked faster than a high-chromium grade, because its chromium content is modest and its molybdenum does not help under oxidising conditions. In hot concentrated sulfuric acid its performance is limited. It is also a difficult material to fabricate economically: it work-hardens rapidly, it needs high-power welding with careful heat input, and its machinability is poor compared with an austenitic stainless. Those costs belong in the evaluation alongside the price per kilogram.

Alloy 31 (N08031, EN 1.4562) occupies the gap between the super-austenitics and C-276. With roughly 27% chromium, 6-7% molybdenum and a nitrogen addition, it delivers a pitting resistance equivalent in the high forties at a materially lower nickel and molybdenum content than C-276, which makes it attractive in chloride-bearing sulfuric and phosphoric acid where the duty is mixed oxidising-reducing and the temperature is high. Alloy 33 (R20033, EN 1.4591) takes the chromium route further, with 31-35% chromium and around 0.4% nitrogen, and is used where oxidising acid and chloride coexist and C-276's low chromium would be a liability.

The choice inside this family therefore comes down to the redox character of the stream. Reducing or chloride-dominated: C-276. Mixed with a strong oxidising component and high chloride: Alloy 31 or Alloy 33. Purely oxidising with chloride but moderate temperature: a 6Mo grade. Getting this one distinction right is worth more than any amount of price negotiation, because it is the distinction that decides whether the part lasts five years or five months.

Specification systems differ across this family, and the cross-reference below is provided so that a European or Chinese specification can be read against the ASTM document without losing a requirement. Designations are given as chemistry-based equivalences where no exact grade exists.

Grade (UNS)	US - ASTM / ASME	GB - GB/T	JIS	DIN / EN (W.Nr)	GOST
Alloy 20 (N08020)	ASTM B463, B473, B729; ASME SB463	GB/T 15007 NS-series (by chemistry)	JIS G 4901 / G 4902 / G 4903	DIN 17744; EN 2.4660	GOST 5632 (by chemistry)
Alloy 28 (N08028)	ASTM B668	GB/T 15007 NS-series (by chemistry)	JIS G 4903 (by chemistry)	EN 1.4563	GOST 5632 (by chemistry)
904L (N08904)	ASTM B625, B677, B649; ASME SB625	GB/T 20878 015Cr21Ni26Mo5Cu2	JIS G 4304 / G 4305	EN 1.4539	GOST 5632 (by chemistry)
6Mo (S31254)	ASTM A240, A312	GB/T 20878 (6Mo grade, by chemistry)	JIS G 4304 / G 4305	EN 1.4547	GOST 5632 (by chemistry)
6Mo (N08367)	ASTM B688, A240	GB/T 20878 (by chemistry)	JIS G 4304 / G 4305	EN 1.4529	GOST 5632 (by chemistry)
Alloy 31 (N08031)	By agreement with mill	GB/T 15007 (by chemistry)	JIS G 4901 to 4904 (by chemistry)	EN 1.4562	GOST 5632 (by chemistry)
Alloy 33 (R20033)	By agreement with mill	GB/T 15007 (by chemistry)	JIS G 4901 to 4904 (by chemistry)	EN 1.4591	GOST 5632 (by chemistry)
C-276 (N10276)	ASTM B575, B622, B619, B574; ASME SB575	GB/T 15007 NS3304	JIS G 4901 to 4904 (NW0276)	DIN 17744; EN 2.4819	GOST 5632 (by chemistry)
316L (S31603)	ASTM A240, A312, A480	GB/T 20878 022Cr17Ni12Mo2	JIS G 4304 / G 4305	EN 1.4404	GOST 5632 03Kh17N14M2

Table note: GB/T 15007 covers wrought corrosion-resistant and heat-resistant superalloy grades; GB/T 20878 covers stainless steel designations. Where a cell reads "by chemistry", no single designation in that system corresponds exactly to the UNS grade, and the mill certificate plus a chemical analysis to ASTM E1476 or an agreed PMI procedure should be the acceptance basis. Russian designations are transliterated from GOST 5632.

Anodic Protection, Velocity Limits and Erosion in Acid Circuits

Anodic protection is the one control measure that can change the alloy selection rather than merely support it. By holding the metal potential inside the passive range with a potentiostat, a reference electrode and a cathode, it allows a low-alloy stainless steel to operate in dilute sulfuric acid that would otherwise require Alloy 20 or Alloy 28.

The technique is well established in sulfuric acid service, particularly on carbon steel and 316L storage tanks, acid coolers and concentration equipment. It works because the acid is a conducting electrolyte, which is exactly the condition that makes electrochemical protection practical, and because the passive current density of stainless steel in acid is low enough for a modest instrument to maintain. A well-run anodic protection system can hold 316L passive in dilute acid over a range of concentrations and temperatures where unprotected 316L would corrode rapidly, and it does so at a capital cost that is often a fraction of the alloy upgrade.

The conditions are strict, and they are the reason anodic protection is not universal. The electrolyte must be continuously conductive, so the technique is unsuitable for dry or intermittent service. The potential must be controlled at every wetted surface, which means crevices, dead legs and poorly positioned reference electrodes create unprotected zones that corrode faster than they would without the system. The instrument loop must be reliable, with fail-safe logic that alarms and, where necessary, isolates the equipment if control is lost, because losing protection on a passivated surface can produce rapid attack. And the design must avoid galvanic coupling to a large cathode area, which can drive the protected surface into an over-protected or transpassive condition.

Where those conditions can be met, anodic protection converts a materials problem into an instrumentation problem, and the economics usually favour it for large tanks and coolers. Where the circuit is intermittent, contaminated with chlorides, or geometrically complex, the alloy route remains the safer choice. A hybrid is common: anodic protection on the large tank, Alloy 20 or 28 on the pump, piping and heat exchanger surfaces where the geometry defeats potential control.

Velocity limits in acid circuits are lower than most people expect, because the passive film in acid is thin and the consequence of losing it is severe. The table below summarises typical practice; the values are operating experience, not standard limits, and every circuit should be validated against its own solids content and geometry.

Service	Typical velocity ceiling (m/s)	Governing mechanism	Mitigation
Carbon steel in 96-99% H2SO4	Below 1.0	Sulphate film removal, erosion at the film	Low-speed pumps, generous line sizes, no abrupt contractions
316L or 904L in dilute acid	1.0-1.5	Film shear, especially at welds and bends	Long-radius bends, smooth weld dressing
Alloy 20 in acid with traces of solids	1.5-2.5	Film shear plus particle impingement	Solids removal, thicker wall at the inlet
Alloy 28 in hot dilute acid	1.5-3.0	Film instability at temperature	Temperature control, flow distribution
6Mo and Alloy 31 in chloride acid	2.0-3.5	Pitting initiation at disturbed flow	Avoid deposits, crevice-free design
C-276 in chloride or fluoride acid	3.0-4.5	Impingement at jet and inlet zones	Inlet spargers, impingement plates

Table note: values are typical plant practice and are not standard limits. Acceptance should be based on wall-thickness trend data and, where the duty is marginal, on flow-loop or jet impingement testing; ASTM G73 provides a liquid impingement framework and ASTM G31 a general immersion framework for comparing candidate grades.

Erosion in acid circuits concentrates at a small number of predictable locations: the acid inlet nozzle where a sparger discharges into a tank, the downstream side of a control valve, the extrados of the first bend after a pump, and the impingement plate or distributor in a cooler. In each case the local velocity or the local angle is much higher than the bulk line velocity, and the passive film is destroyed faster than it can reform. The practical countermeasures are geometric before they are metallurgical: an impingement plate at the inlet, a larger nozzle, a sparger designed for low exit velocity, a long-radius bend, and a valve selected for the flashed condition rather than the inlet condition.

Welding, Post-Weld Condition and Intergranular Corrosion Testing

Welding is where most acid-service failures begin, and the reason is metallurgical rather than mechanical. Every austenitic and nickel-based grade in this guide relies on chromium in solution to form its passive film, and heating into the carbide precipitation range removes chromium from solution at the grain boundaries. The result is a narrow chromium-depleted zone that corrodes preferentially in acid, producing the weld decay pattern that has ended more acid plant piping than any grade-selection error.

The remedy is condition of supply, not filler metal choice. Solution annealing dissolves the precipitates and restores homogeneity, and rapid cooling prevents them from re-forming. For fabricated equipment the sequence matters: form, weld, then solution anneal where the specification and geometry allow, or use a stabilised grade or a low-carbon grade and control heat input carefully where a post-weld solution anneal is impractical. Alloy 20's niobium addition and C-276's very low carbon both exist for this reason, but neither removes the need to control interpass temperature and heat input.

Pickling and passivation after fabrication are part of the same requirement. Welding leaves a heat-tinted oxide that is chromium-depleted and, in acid service, is an initiation site for pitting. Mechanical cleaning followed by a properly controlled pickle and passivate restores the surface chemistry; a wire-brushed weld is not an acceptable substitute. For chloride-bearing duties the surface condition matters even more, because a residual iron contaminant from carbon steel tooling becomes the anode in a small galvanic cell.

Verification is by test, not by certificate wording. ASTM G28 covers the detection of susceptibility to intergranular corrosion in wrought nickel-rich chromium-bearing alloys; Practice A is the screening test normally applied to Ni-Cr-Mo grades such as C-276, and Practice B is applied to Ni-Fe-Cr grades such as Alloy 20. Acceptance criteria differ between practices and between standard revisions, so the purchase order should quote the practice, the revision and the mass-loss limit explicitly. For the stainless grades, ASTM A262 Practice E is the classical sensitisation check applied to 904L and 316L, with Practice C used in some specifications.

Grade	Condition of supply	Solution anneal (typical)	Cooling	Corrosion / sensitisation verification
Alloy 20	Solution annealed, pickled	925-1010 °C	Rapid water quench	ASTM G28 Practice B; ASTM A262 Practice E where specified
Alloy 28	Solution annealed	1050-1120 °C	Water quench	ASTM G28 Practice B; ASTM A262 Practice E
904L	Solution annealed, pickled	1040-1120 °C	Water quench	ASTM A262 Practice E; ASTM G48 for pitting
6Mo (S31254 / N08367)	Solution annealed, pickled	1150-1200 °C	Rapid water quench	ASTM A262 Practice E; ASTM G48 Practice A or C
Alloy 31	Solution annealed	1150-1200 °C	Water quench	ASTM G28 Practice A or B as agreed
C-276	Solution annealed, rapid cooled	1120-1170 °C	Water quench or rapid gas	ASTM G28 Practice A
316L	Solution annealed, pickled	1040-1120 °C	Water quench	ASTM A262 Practice E; ASTM G48 for pitting

Table note: annealing ranges are typical mill practice and are not standard limits; the controlling values are those in the applicable product specification and the agreed mill route. Test acceptance criteria must be stated numerically in the purchase order, with the standard revision identified, because G28 and A262 criteria are revision-dependent.

Indicative 2026 Price Reference for Sulfuric Acid Grades

The grades in this guide span a wide price range, and the difference between them is often larger than the difference between two suppliers quoting the same grade. The table gives indicative 2026 EXW Shanghai price bands for plate so that the cost of moving up the selection ladder can be estimated before the design is frozen.

Grade	Indicative price, 2026, EXW Shanghai, USD/kg	Relative to 316L	Typical sulfuric acid role	Standard
904L	18-28	3.5	Aerated dilute acid, no chlorides	ASTM B625 / A240
6Mo super-austenitic	22-32	4.5	Dilute acid with moderate chloride	ASTM B688 / A240
Alloy 20	30-45	6.0	Hot dilute acid with chlorides or fluorides	ASTM B463
Alloy 28	35-50	7.0	Hot dilute acid, higher temperature	ASTM B668
Alloy 31	35-50	7.0	Mixed acid, chloride bearing	ASTM B625
C-276	65-95	12.0	High chloride, reducing, mixed acid	ASTM B575
Type 316L (reference)	4-8	1.0	Not for hot dilute acid	ASTM A240

Table note: 2026, EXW Shanghai, USD/kg, indicative ranges only, subject to nickel and molybdenum price movement, quantity and test package. Relative cost uses the midpoint of the 316L range. Not an offer.

Standard Index

Standard	Title or scope	Covers	Product form
ASTM B463	Alloy 20 (N08020) plate, sheet and strip	Chemistry, mechanicals, condition	Plate, sheet, strip
ASTM B473	Alloy 20 bar, rod and wire	Chemistry, mechanicals, finish	Bar, rod, wire
ASTM B668	Alloy 28 (N08028) seamless tube and pipe	Chemistry, mechanicals, pressure rating	Seamless tube, pipe
ASTM B575	Ni-Mo-Cr low-carbon plate, sheet and strip (C-276, C-22, C-2000)	Chemistry, mechanicals, condition	Plate, sheet, strip
ASTM B622	Ni-Mo-Cr seamless tube and pipe	Chemistry, mechanicals, pressure rating	Seamless tube, pipe
ASTM B625	Ni-Fe-Cr-Mo-Cu low-carbon plate, sheet and strip (904L, 254SMO family)	Chemistry, mechanicals, condition	Plate, sheet, strip
ASTM G28	Detecting susceptibility to intergranular corrosion in wrought nickel-rich chromium-bearing alloys	Practice A (Ni-Cr-Mo), Practice B (Ni-Fe-Cr)	Wrought products
ASTM A262	Detecting susceptibility to intergranular attack in austenitic stainless steels	Practice E copper-copper sulphate test	Wrought stainless
ASTM G48	Pitting and crevice corrosion resistance by ferric chloride	Practice A, C, D, E	Wrought products
ASTM G31	Laboratory immersion corrosion testing	Rate calculation and reporting	Coupons
ASTM G73	Liquid impingement erosion testing	Erosion rate ranking	Coupons
EN 17744	Nickel alloys, chemical composition	Grade composition	All forms
EN 17750	Nickel wrought alloys, plate, sheet and strip	Properties and tolerances	Plate, sheet, strip
GB/T 15007	Designation of corrosion-resistant alloys	Chinese grade designation	All forms
JIS G 4901	Corrosion-resistant and heat-resistant superalloy bars	Chemistry, mechanicals	Bar
JIS G 4902	Corrosion-resistant and heat-resistant superalloy plates	Chemistry, mechanicals	Plate
GOST 5632	Corrosion-resistant and heat-resistant alloy grades	Grade designation and composition	All forms

Table note: the standards above are the ones a buyer will meet on a sulfuric acid enquiry. Product specifications set chemistry and mechanicals; test methods set the acceptance evidence. Always quote the edition and, for G28 and A262, the practice letter and the acceptance limit in the purchase order.

The relationship between grade and standard is not one-to-one across systems, and a sulfuric acid enquiry that mixes them is a common source of quotation error. Alloy 20 is N08020 in the UNS system, 2.4660 in the DIN workshop designation, and has a Chinese equivalent that is close but not compositionally identical; 904L is N08904 and 1.4539; C-276 is N10276 and 2.4819. A buyer who specifies "Alloy 20 to GB" without naming the standard will receive a grade that meets the Chinese designation and may sit at a different point in the molybdenum and copper ranges. The safe practice is to quote the UNS number and the ASTM product specification, and to treat any cross-reference table as a starting point for discussion rather than as an equivalence. The GB versus ASTM cross-reference guide sets out how the two systems differ in structure, and the alloy knowledge centre carries the composition tables behind these designations.

FAQ: Sulfuric Acid Alloy Selection

Q1: Which alloy should I specify for 98% sulfuric acid at ambient temperature?

At 98% concentration and ambient temperature, carbon steel is normally the correct and most economical material, provided the acid is kept above about 93% and the velocity is controlled. Concentrated sulfuric acid in this range is strongly oxidising and passivates steel; the corrosion rate is low enough that carbon steel tanks, tankers and piping are standard practice worldwide. The danger is not the nominal concentration but the transitions: dilution at a water injection point, condensation at a breather, or a shutdown wash that leaves dilute acid in contact with the steel. Those transitions are where carbon steel fails, and they are the reason concentrated acid systems are designed with dilution control, dry breathers and defined wash procedures rather than with a more expensive alloy. If the duty includes frequent dilution, a temperature above about 40 °C, or chloride contamination, the selection moves to a stainless or nickel alloy. Alloy 20 and 904L are used in the transition zones, and C-276 where chlorides are present. The mistake to avoid is specifying an expensive alloy for the bulk of a 98% system while leaving the transition points in carbon steel.

Q2: Why is dilute sulfuric acid more aggressive than concentrated acid?

Dilute sulfuric acid is aggressive because it is a strong acid with a high hydrogen ion activity and, at the same time, an insufficient oxidising potential to maintain a passive film on most metals. In concentrated acid the sulphate ion acts as an oxidiser and steel passivates; in dilute acid the same steel is in the active state and dissolves at a rate that rises steeply as concentration falls below about 70% and as temperature rises. The worst region is usually between about 20% and 60% at elevated temperature, where both the acid activity and the anodic reaction rate are high and no passive film is stable. Aeration changes the picture: dissolved oxygen can raise the potential into the passive range for stainless grades, which is why 904L and the 6Mo austenitics perform well in aerated dilute acid and poorly in deaerated acid. This is also the basis of anodic protection, which artificially raises the potential to hold the alloy passive. Any selection based on concentration alone, without stating temperature, aeration and chloride level, is incomplete.

Q3: Where does Alloy 20 outperform 904L in sulfuric acid?

Alloy 20 outperforms 904L where the acid contains chlorides and where the duty is hot dilute sulfuric acid with mixed acid contamination. The copper addition in Alloy 20 raises its resistance to sulfuric acid specifically, while the molybdenum and nickel content provide resistance to chloride pitting and to the reducing conditions that appear when the acid is contaminated. 904L is a excellent general-purpose austenitic for aerated dilute acid and is cheaper, but its molybdenum content is lower and it has no copper, so in hot 40-60% acid with chlorides its corrosion rate climbs faster. Alloy 20 also has better resistance to sulfuric acid contaminated with fluorides, which is why it became the standard material in wet-process phosphoric acid plants where sulfuric acid is the attacking species. The practical decision rule is: aerated dilute acid without chlorides, use 904L; hot dilute acid with chlorides or fluorides, use Alloy 20; and if the acid is also reducing or the chloride level is high enough to pit Alloy 20, move to Alloy 28 or C-276.

Q4: When do I need C-276 instead of Alloy 28?

C-276 is needed when the sulfuric acid duty includes high chlorides, strong reducing conditions, or contamination that pushes the environment outside the envelope of the austenitic grades. Alloy 28 is a high-chromium, high-molybdenum austenitic developed for hot sulfuric and phosphoric acid, and it performs well in dilute acid up to moderate chloride levels. C-276, with roughly 15 to 17% molybdenum and a low carbon content, tolerates much higher chloride concentrations and resists both oxidising and reducing acids, which makes it the choice for mixed acid, for chloride-contaminated spent acid, and for areas where upsets can produce reducing conditions. The cost difference is substantial, often a multiple of the Alloy 28 price, so C-276 should be used where the duty genuinely requires it rather than as a default. A useful test is to obtain a chloride analysis over the full operating cycle rather than a single spot sample: many plants discover that chloride peaks during regeneration or cleaning, and those peaks decide the grade. Where the chloride peak is unknown, a coupon test in the actual liquor is cheaper than an upgrade.

Q5: Does anodic protection replace alloy selection?

Anodic protection does not replace alloy selection, but it changes the economics of it. The technique holds the metal at a potential in the passive range by passing a small current through the acid, which can reduce the corrosion rate of an otherwise marginal grade by orders of magnitude. It works best in concentrated sulfuric acid and in oleum, where the electrolyte is sufficiently conductive and the passive range is wide, and it is widely used in acid coolers, storage tanks and rail tankers. What it does not do is protect areas that are not wetted, crevices, or the vapour space, and it fails if the acid is diluted below the conductivity needed to carry the protective current. It also adds a control system, reference electrodes and a power supply that must be maintained. The correct framing is that anodic protection allows a cheaper alloy or carbon steel to be used in a well-defined, continuously wetted, concentrated-acid duty, provided the dilution and shutdown cases are engineered. It is not a substitute for choosing a corrosion-resistant alloy in dilute or chloride-bearing service.

Q6: How do chlorides change the selection in a sulfuric acid plant?

Chlorides change the selection because they break down the passive film locally and cause pitting and crevice corrosion at temperatures and concentrations where the bulk acid would otherwise be harmless. In sulfuric acid service, chloride is often present as a trace contaminant from raw materials, from cooling water ingress, or from carryover in a scrubbing circuit, and it concentrates in dead legs, under deposits and at the water line. The effect is not linear: a few hundred parts per million can be enough to pit 904L at elevated temperature, while Alloy 20 tolerates more and C-276 tolerates more again. The practical approach is to establish the chloride level at the worst point of the cycle, not the average, and to check it at the locations where concentration can occur. Where chlorides are unavoidable and the temperature is high, the selection moves to C-276 or to a super-austenitic with a high PREN, and the design should eliminate crevices and dead legs because chlorides concentrate there first.

Q7: What corrosion test should I require for sulfuric acid service?

The test depends on the failure mode you are trying to prevent, and for sulfuric acid service the two that matter most are intergranular corrosion and pitting. ASTM G28 Practice A is the standard screening test for Ni-Cr-Mo grades such as C-276, and Practice B for Ni-Fe-Cr grades such as Alloy 20 and Alloy 28; both measure mass loss after exposure to a boiling sulfuric acid-ferric sulphate solution and are used to detect sensitisation and improper heat treatment. For chloride pitting, ASTM G48 with ferric chloride is the usual ranking test, although it is more aggressive than most sulfuric acid duties and should be used to compare grades rather than to predict service life. Where the actual liquor is available, ASTM G31 immersion testing in the real acid at the real temperature gives the most relevant rate, and ASTM G73 covers impingement where velocity is high. The purchase order should name the practice, the revision and the numerical acceptance limit; a certificate that says "passed G28" without a limit is not verifiable.

Q8: Is Alloy 20 suitable for oleum?

Alloy 20 is not the normal choice for oleum. Oleum, or fuming sulfuric acid, contains dissolved sulphur trioxide and is an extremely strong oxidising and dehydrating medium; it is normally handled in carbon steel because the steel forms a protective sulphate film, and the selection question is usually about the transition zones rather than the bulk. Where oleum is diluted to concentrated acid, the dilution point experiences heat and concentration transients that can be aggressive to both steel and stainless, and the material at that point is chosen for the transient rather than the bulk. Alloy 20 has limited resistance to strongly oxidising conditions and is not the grade to specify for oleum or for the vapour space above it. If a nickel alloy is required in an oleum system, it is usually for a specific component such as a pump or a valve where velocity and turbulence prevent the protective film from forming, and the selection should be made with the acid supplier and a corrosion test rather than from a general table.

Q9: How do I weld Alloy 20 for sulfuric acid service?

Weld Alloy 20 with a matching filler and a procedure qualified to ASME Section IX, with heat input controlled to avoid sensitisation and with a post-weld clean-up that removes heat tint. The grade is stabilised with niobium and has a low carbon content, which reduces but does not eliminate the risk of chromium carbide precipitation in the heat-affected zone; the practical controls are low heat input, interpass temperature control, and where geometry allows, a post-weld solution anneal. The weld metal itself is usually more resistant than the HAZ because it is fully austenitic with a higher nickel content, so failures tend to occur just outside the fusion line. After welding, the oxide tint must be removed by pickling or by mechanical means followed by passivation, because the tint is chromium-depleted and is an initiation site in acid. For critical welds, a G28 Practice B test on a procedure coupon gives direct evidence that the procedure produces a resistant HAZ.

Q10: Why do acid plant pipes fail at welds more often than in the parent metal?

Acid plant pipes fail at welds because the weld and its heat-affected zone are where composition, microstructure and residual stress all depart from the parent metal at the same time. The HAZ experiences a thermal cycle that can precipitate chromium-rich carbides at grain boundaries, leaving a narrow chromium-depleted zone that corrodes preferentially in acid. The weld also retains residual tensile stress, which combine with the corrosive environment to drive stress corrosion cracking in chloride-bearing duties. Geometrically, a weld root protrusion or an internal mismatch creates a local flow disturbance where velocity and turbulence rise, accelerating corrosion and erosion. The remedies are metallurgical and procedural: qualified procedures with controlled heat input, matching filler, post-weld solution anneal where possible, and pickling and passivation to restore the surface. Inspection should focus on the HAZ rather than the weld metal, and where the duty is critical, a corrosion test on a procedure coupon is more informative than a visual inspection.

Q11: What documentation should accompany a sulfuric acid alloy order?

The order should be accompanied by an EN 10204 3.1 mill test certificate that reports actual chemical composition and mechanical properties for each heat, a heat treatment record showing the solution anneal temperature and cooling method, and the results of the corrosion test specified in the order with the practice letter and acceptance limit. For pressure equipment, the certificate must also reference the design code and the applicable product specification, and for welded components the welding procedure qualification and the welder qualification records should be available. Dimensional and surface inspection records matter in acid service because a surface defect or a residual heat tint is an initiation site, so the certificate should state the surface condition and the pickling and passivation status. Where the buyer has specified ultrasonic or penetrant testing, the reports should be attached. Hangbo Alloy issues this documentation package as standard for acid-service orders and can add third-party witnessing by SGS, BV or TUV where the project requires it.

Conclusion: Select on the Worst Point of the Cycle, Not the Datasheet Average

Sulfuric acid alloy selection is decided by the concentration, temperature, aeration and chloride level at the worst point of the operating cycle, and by the condition of the material after welding. Carbon steel remains correct for concentrated acid above about 93% at moderate temperature; 904L is the economic choice for aerated dilute acid without chlorides; Alloy 20 is the benchmark for hot dilute acid containing chlorides or fluorides; Alloy 28 extends that envelope upward in temperature and chloride; and C-276 is reserved for high-chloride, reducing or genuinely mixed-acid duties where the austenitic grades cannot hold a passive film. The most expensive mistake in the sequence is not choosing the wrong alloy for the bulk duty — it is leaving the dilution point, the vapour space or the weld HAZ in a material that cannot survive the transient.

The engineering programme that prevents that mistake is short. Map concentration and temperature across the whole cycle, including start-up, shutdown, washing and upset. Measure chloride at the points where it concentrates, not at the pump discharge. Select the grade against the worst combination, then confirm the condition of supply with a solution anneal and a G28 test. Qualify the welding procedure with heat input control, and pickle and passivate every weld. Specify the documentation before the order is placed. Buyers who want to compare the candidate grades against a specific liquor can review the Hastelloy product family and the Incoloy product family, or send the operating envelope to the sales engineering desk for a written recommendation with the applicable test programme.

Shanghai Hangbo Alloy Group supplies Alloy 20, Alloy 28, 904L, the 6Mo super-austenitics and C-276 in plate, sheet, bar, seamless tube and welded pipe, with solution-annealed condition, G28 testing and EN 10204 3.1 certification as standard for acid service.

Related Reading

1. Hastelloy C-276 versus Incoloy 825 in acid and chloride service
2. FGD scrubber alloy selection guide

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Shanghai Hangbo Alloy Group Co., Ltd. - China Supplier

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