Nickel Alloy Erosion Corrosion Failure Analysis Guide (54 chars)

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

Contact: sales@hangboalloy.com | hangbo@nickel-alloy.com | WhatsApp (Lisa): +86 13611656360

Quick Answer: Erosion-Corrosion Diagnosis and Prevention

Erosion-corrosion is diagnosed by mapping wall-thickness loss against flow direction, impingement angle and solids loading. Diagnosis combines ASTM G119 synergy testing, ASTM G73 impingement data and ASTM G40 surface descriptors. Prevention means holding velocity below the alloy threshold, matching composition to the slurry chemistry and overlaying high-angle zones.

Key Takeaways

• Erosion-corrosion is synergistic, not additive. Total loss in a slurry line typically exceeds the sum of pure erosion plus pure corrosion, because material removal continuously exposes fresh, unpassivated metal.
• Velocity, not chemistry alone, decides service life. Above a threshold that is alloy- and solids-specific, protective passive films are destroyed faster than they can repassivate.
• Angle of impingement changes the dominant mechanism. Shallow angles favour cutting and corrosion-assisted grooving; normal or steep angles favour brittle cracking and pitting.
• C-276 outperforms 316L in reducing and chloride-bearing slurries; Alloy 20 outperforms 316L in phosphoric acid service. Both resist erosion-corrosion better than 316L mainly through molybdenum, chromium and copper content.
• Weld overlay and hard-facing are the fastest retrofit. Cobalt- or nickel-based overlays applied at impingement zones cut local loss dramatically without replacing the whole component.
• A returned part carries the evidence. Flow-directional grooves, comet-tail pits, weld-cone attack and bright freshly exposed metal tell you the mechanism before any metallography begins.

Why Erosion-Corrosion Fails Nickel Alloys in Slurry and High-Velocity Service

Erosion-corrosion is the accelerated loss of material caused by the combined action of a flowing corrosive medium and mechanical impact or shear from suspended solids. It finds nickel alloys in exactly the places where pumps, valves and piping move abrasive slurries under pressure, and it kills components that were correctly selected on chemistry alone.

The important point is that the two mechanisms feed each other. Where a passive film is stable, corrosion contributes almost nothing; the mechanical damage rate dominates. Where the film is repeatedly destroyed, the anodic reaction runs on clean metal, and the corrosion contribution climbs steeply. The measured total loss therefore rises far faster than either mechanism alone would predict, and the failure arrives earlier than a coupon test in still liquid would suggest.

Nickel alloys are popular in these services because they resist the corrosive half of the problem. Hastelloy C-276 and other Ni-Mo-Cr grades withstand reducing acids and chloride-rich liquors, Alloy 20 handles sulphuric and phosphoric acid with acid-soluble chlorides, and the higher-molybdenum super-austenitics broaden that envelope further. What they do not do is become hard. Bulk hardness on these grades typically sits well below that of a martensitic white iron or a ceramic, so once a passive film is stripped, the metal surface is easy mechanical prey.

That creates a distinctive failure signature: a component that corroded almost nothing in a static test, and disappears within months in a circulating slurry. The diagnosis job is to quantify how much of the loss was corrosion, how much was erosion, and how much was the synergy between them. ASTM G119 provides the framework for that accounting: static corrosion rate, erosion-only rate in an inert slurry, and the combined rate in the real environment, from which a synergy ratio is calculated. When the ratio exceeds roughly 1.5 to 2, the design must be treated as an erosion-corrosion problem rather than a corrosion problem, and mitigation shifts from alloy chemistry toward flow control and surface hardening. Any nickel alloy product family can be assessed this way before a replacement decision is taken.

Diagnosing a Returned Part: Reading the Damage Without Metallography First

A returned pump impeller, let-down valve trim or pipe elbow answers most of the diagnostic questions before a single micrograph is taken, because the damage geometry encodes the mechanism. Read the surface first, then use metallography only to confirm the reading.

Start by locating the maximum wall loss and measuring its angular relationship to the local flow. If the deepest loss sits inside an elbow extrados, downstream of a weld root protrusion or at a valve seat where a jet impinges, you are looking at impingement-driven erosion-corrosion rather than general corrosion or galvanic attack. If loss is spread evenly along a straight run with a polished appearance, general flow-accelerated corrosion is more likely. Then look at the pattern within the worn zone.

Flow-directional grooves, sand-polished facets and comet-tail pits clustering behind each groove are the classic combination. The grooves show the cutting action of particles at shallow angles; the comet-tail pits are where a particle disrupted the passive film and left a small anodic cell behind. Bright, freshly exposed metal inside the grooves means the film never had time to reform. A dark, tightly adherent oxide with undercut edges, by contrast, points to an acidic or chloride-driven local corrosion cell that was already active before the solids arrived. Layered or lamellar tearing at grain boundaries indicates that selective phase attack accompanied the mechanical loss, which is common where second phases are less corrosion resistant than the matrix.

Three further observations narrow the diagnosis. First, whether the part failed at a weld or in parent metal: attack concentrated at the weld cone, HAZ or overlay boundary suggests a composition or dilution problem rather than a flow problem. Second, whether the loss is depth-limited or geometry-limited: a scalloped, polished, concave wear face that keeps its overall shape indicates uniform mechanical removal, while deep isolated pits indicate local corrosion having the upper hand. Third, whether deposits are present: a scale ledge or a fibre or sand deposit creates a crevice, and crevices concentrate erosion-corrosion because the local geometry accelerates the flow that just passed over the deposit. Every one of these observations should be logged with a profile trace across the worn zone, because the profile shape is what distinguishes material removal by cutting from removal by corrosion-assisted undermining.

Finally, quantify. Wall-thickness mapping, ASTM G40 terminology for the surface, and a measured loss rate in millimetres per year, compared with whatever baseline the original design assumed, convert a visual impression into a decision. If loss rate is above the design allowance and the worn zone is flow-aligned, the fix is a hydraulic or materials change, not another identical replacement. The failure analysis case notes published from slurry and acid plant service illustrate how often a returned part shows the same three signatures: shallow-angle grooving, comet-tail pitting and weld-cone attack.

Velocity Limits, Solids Loading and the Threshold That Actually Governs Life

Every nickel alloy in slurry service has an effective velocity limit, and above it service life collapses far faster than a linear relationship would suggest. The limit is not a single published number; it is a function of alloy, slurry chemistry, solids concentration, particle size and geometry.

Above the threshold, the passive film is destroyed at a rate exceeding the repassivation rate. Once that happens, the surface behaves as bare metal, and the corrosion contribution becomes large. Practical plant experience with austenitic and nickel alloys in neutral or mildly acidic slurries usually places the onset of severe attack somewhere in the range of 3 to 5 m/s for high-solids slurries, shifting upward for low solids and clean liquid, and downward sharply in acidic liquors or where sand is present. These are typical operating observations, not standard minima, and they must be validated per circuit.

Solids concentration changes the picture in a non-linear way. Increasing sand from 1% to 5% by weight does not double the loss rate; the number of impacts per unit area rises, and perturbed flow near each impacted zone reduces the local film repair rate, so the measured loss can triple or worse. Particle size matters in a similar direction, but the responsible variable is more often particle momentum, so a smaller population of large, hard particles can do more damage than a larger population of fines. Above roughly 200 to 300 micrometres in hard mineral slurries, impingement damage becomes the dominant term in most nickel alloys operating above 3 m/s.

Angle of impingement splits the failure mode. At shallow angles, typically up to about 15 to 20 degrees from the surface, ductile materials lose metal by a cutting and ploughing action, producing directional grooves. At steep to normal angles, typically 60 to 90 degrees, the mechanism shifts toward repeated plastic deformation, cracking and material detachment, and brittle or heavily cold-worked surfaces suffer disproportionately. Many nickel alloys are relatively ductile in the annealed condition, so they survive glancing-angle flow better than they survive a direct jet. This is precisely why valve seats, orifice plates, pump cutwaters and the outer radius of elbows fail first: they convert a glancing flow into a near-normal jet.

Geometry should therefore be treated as a design variable with the same weight as alloy selection. Reducing the number of sharp changes in direction, avoiding abrupt expansions where a jet forms, increasing elbow radius, eliminating weld root protrusions that shed vortices, and using low-velocity bypass lines around control valves all lower the local impingement angle or velocity. Where geometry cannot be changed, the alloy or surface condition must be, and the machined and forged product forms that allow single-piece components without internal weld geometry are often the most economical answer.

C-276, Alloy 20 and 316L: Why the Same Slurry Treats Them Differently

The reason C-276 and Alloy 20 survive slurries that destroy 316L is not hardness and not strength; it is the stability of the passive film and the speed at which it reforms after an impact. Composition sets that stability, and the differences are large enough to change failure mode, not just service life.

Molybdenum is the single most influential addition. In reducing acids and chloride-bearing liquors, molybdenum promotes repassivation and suppresses active dissolution, so a freshly gouged surface in C-276 begins to rebuild its film immediately after the particle leaves. Chromium supplies the film itself; nickel stabilises the austenitic matrix and, in the nickel-based grades, keeps the alloy ductile and metallurgically stable at high molybdenum levels. In Alloy 20, copper is the distinguishing addition, and it is what gives the grade its resistance to sulphuric acid and to the mixed acid conditions found in phosphoric acid plants. Type 316L has none of these advantages at the same level: its molybdenum content is roughly a quarter of C-276's and it contains no copper at all, so in the same impingement zone it strips, dissolves and repassivates more slowly, and the corrosion half of the synergistic loss grows.

Alloy	Standard	Ni %	Cr %	Mo %	Cu %	Other (typical)
Hastelloy C-276 (UNS N10276)	ASTM B575	Balance	14.5-16.5	15.0-17.0	-	W 3.0-4.5, Fe 4.0-7.0, Co 2.5 max
Alloy 20 (UNS N08020)	ASTM B463	32.0-38.0	19.0-21.0	2.0-3.0	3.0-4.0	Nb 8xC-1.0, Fe balance
Incoloy 825 (UNS N08825)	ASTM B424	38.0-46.0	19.5-23.5	2.5-3.5	1.5-3.0	Ti 0.6-1.2, Fe 22.0 min
Inconel 625 (UNS N06625)	ASTM B443	58.0 min	20.0-23.0	8.0-10.0	-	Nb+Ta 3.15-4.15, Fe 5.0 max
Super austenitic (UNS N08367 / S31254)	ASTM B688 / A240	23.5-25.5	20.0-22.0	6.0-7.0	0.5-1.0	N 0.18-0.25, PREN above 40
Type 316L (UNS S31603)	ASTM A240	10.0-14.0	16.0-18.0	2.0-3.0	-	Fe balance, C 0.030 max

Mechanical strength matters less than ductility in erosion-corrosion, but the numbers still matter when a designer is choosing between a heavier section and a stronger one. Nickel-based grades generally show higher yield strength than 316L at both ambient and elevated temperature, which allows thinner walls for the same pressure rating, but thinner walls mean less sacrificial thickness once wear begins. In practice, most slurry components are thickness-limited rather than stress-limited, so designers should specify the corrosion allowance first and the strength second.

Alloy	Standard	Tensile (typical, MPa)	0.2% Yield (typical, MPa)	Elongation (typical, %)	Hardness (typical, HB)
Hastelloy C-276	ASTM B575 / E8	790	355	40	185-200
Alloy 20	ASTM B463 / E8	620	240	30-45	140-180
Incoloy 825	ASTM B424 / E8	690	310	30	140-190
Inconel 625	ASTM B443 / E8	830	415	30	150-220
Super austenitic 6Mo	ASTM B688 / A240	690	310	30	190-230
Type 316L	ASTM A240 / E8	485	170	40	130-180

Heat treatment is the overlooked variable in erosion-corrosion failures, because the wrong thermal condition changes both phase structure and residual stress. Solution annealing dissolves secondary phases and restores the single-phase austenitic structure that has the best film repair behaviour. Sensitised material, or a component that saw a slow cool after welding, can contain chromium-depleted zones at grain boundaries; those zones corrode preferentially and the resulting intergranular trenches become mechanical stress raisers that erosion then deepens. Purchasing to a specification that includes a solution anneal and a sensitisation test is therefore a genuine erosion-corrosion countermeasure, not a formality. EN 10204 3.1 or 3.2 documentation should confirm condition of supply for critical trim.

Alloy	Standard covering condition	Solution anneal (typical)	Cooling	Sensitisation / acceptance test
Hastelloy C-276	ASTM B575	1120-1170 °C	Water quench / rapid	ASTM G28 Practice A, ASTM A262 where applicable
Alloy 20	ASTM B463	925-1010 °C	Water quench	ASTM A262 Practice E, ASTM G28
Incoloy 825	ASTM B424	925-1010 °C	Water quench	ASTM G28 Practice B
Inconel 625	ASTM B443	1090-1200 °C	Water quench / air	ASTM G28, hardness check
Type 316L	ASTM A240 / A480	1040-1120 °C	Water quench / rapid	ASTM A262 Practice E, G48 for pitting
Weld overlay (Ni-based)	ASTM B575 / ASME IX	Procedure-qualified per ASME Section IX	Controlled interpass	ASTM G48 where chloride service applies

A practical consequence: where the same slurry has destroyed both 316L and Alloy 20, the true cause is frequently a combination of alloy and condition rather than alloy alone. Replacing 316L with C-276 while retaining the same sensitised weld procedure simply moves the failure to the weld boundary. Readers comparing grades in more detail can review the corrosion-resistant grade portfolio and the associated grade notes.

Slurry-Specific Behaviour: Phosphoric Acid, Alumina Red Mud and FGD Limestone

Each major slurry service imposes a different balance of chemical and mechanical load, and the corrective action for one is often wrong for another. Phosphoric acid slurry is dominated by acid attack with erosion as an accelerator; alumina red mud is dominated by caustic chemistry combined with very fine, hard solids; FGD limestone slurry is dominated by chlorides, oxygen and moderate solids loading at low-to-moderate pH.

Phosphoric acid slurry, as produced in wet-process phosphoric acid plants, contains sulphuric acid, fluoride, chloride and a high load of phosphate solids, often as gypsum. It is a strongly reducing-to-mixed acid environment at elevated temperature, and it is exactly the chemistry for which Alloy 20 was developed: copper and molybdenum together resist sulphuric acid attack, while the nickel-chromium matrix resists the fluoride-bearing liquors. Attack in this service is usually corrosive first, and erosion accelerates it at agitators, pump impellers, let-down points and the region immediately downstream of flash coolers where entrained solids are moving quickly. Typical field failures show thinning rather than grooving, with the worst loss where temperature, acid concentration and solids velocity all peak together. The mitigation hierarchy is straightforward: keep agitator and pump tip speeds low, increase corrosion allowance, and specify Alloy 20, Alloy 28 or a super-austenitic where chlorides are also high.

Alumina red mud presents the opposite profile. It is strongly alkaline, often above pH 12, and carries a very fine, very hard solid fraction, so the mechanical term is significant even at modest velocities because the fine solids continuously polish the metal and prevent stable film formation. Erosion-corrosion in red mud service appears as broad, smooth, polished wear on impeller shrouds, in pipeline bends and on valve bodies, rather than as deep local pits. Because caustic solutions are aggressive toward silicate-containing refractories and toward aluminium, but comparatively benign to nickel alloys, grade selection usually centres on nickel-rich materials and on controlling solids content by dilution or by upstream desanding. In this service, hardness-based solutions such as overlay coatings frequently outperform an upgrade in alloy chemistry, because the chemistry is already adequate and the failure is almost purely mechanical.

FGD limestone slurry sits between these extremes and is the most common erosion-corrosion environment in coal-fired plants. Limestone or lime slurry typically contains 10 to 30% solids by weight, with chlorides accumulating in closed-loop systems and oxygen entering at absorber surfaces and agitators. Attack concentrates at absorber agitator blades, slurry recirculation pump impellers and casings, spray header nozzles, discharge piping and the wet-dry interface where a partially dry deposit creates crevices. Chloride content drives pitting and crevice corrosion, which then becomes the nucleating site for erosion. Because chloride levels rise as the loop is closed, a material that passed the first year of service can begin failing in the third; the diagnosis should include a chloride and pH history, not just a spot sample taken at the time of failure.

Service	Dominant mechanism	Typical solids	Key chemical driver	Reference standard for test method	Preferred alloy direction
Phosphoric acid slurry	Corrosion with erosion acceleration	20-40% gypsum and phosphate solids	H2SO4, F-, Cl-, elevated temperature	ASTM G48, ASTM G28	Alloy 20, Alloy 28, super austenitic
Alumina red mud	Erosion with alkaline corrosion	5-25% fine hard solids	NaOH above pH 12	ASTM G73, ASTM G32	Nickel-rich grades plus hard-facing overlay
FGD limestone slurry	Chloride pitting plus erosion	10-30% limestone solids	Cl- accumulation, dissolved O2, low pH excursions	ASTM G48, ASTM G119	Super austenitic, C-276 at high chloride
Mining hydrocyclone feed	High-angle impingement erosion	10-40% sand, coarse	Near-neutral to mildly acidic	ASTM G73, ASTM G40	Overlay on Alloy 20 or 316L substrate
Tailings and backfill line	Low-angle cutting erosion	30-60% solids	pH dependent, often neutral	ASTM G119, ASTM G32	C-276 where chloride, overlay where not

Where the actual solids loading is unknown, a site measurement campaign is cheaper than the wrong material. Slurry samples, particle-size distribution and a short-term erosion rate measured with an insert coupon will normally settle the question in weeks. For help interpreting measurements against a grade envelope, the technical contact desk can review sample data alongside the failure photographs.

Purchasing comparisons for 2026 are given below purely as reference ranges, EXW Shanghai, USD/kg. Actual levels move with nickel, molybdenum and cobalt indices and with quantity, form and certification, and no figure here should be treated as a quotation.

Material	Reference price range, 2026, EXW Shanghai, USD/kg (reference only)	Relative erosion-corrosion resistance	Typical forms supplied
Hastelloy C-276 bar	45-70	Very high	Bar, plate, forging, wire
Alloy 20 bar	30-48	High in acid slurries	Bar, tube, plate, fitting
Incoloy 825 bar	26-42	High in mixed acid	Bar, tube, plate
Inconel 625 bar	40-62	Very high, also overlay wire	Bar, overlay wire, plate
Super austenitic 6Mo	18-30	High in chloride	Bar, plate, tube
Type 316L bar	6-11	Moderate; fails above threshold	Bar, plate, tube, pipe
Ni-based overlay wire	38-65	High at impingement zones	Wire, electrode, powder

Failure Zones by Equipment: Pumps, Hydrocyclones and Let-Down Valves

Knowing which zone fails and why shortens the diagnosis dramatically, because each equipment type generates a characteristic local flow field. The same alloy will last years in one of these positions and months in another, purely because of the local velocity vector and angle.

Pump casings fail at the cutwater, the volute throat and the region around the discharge nozzle, because that is where the flow turns sharply and where a recirculation vortex forms. The vortex continuously throws solids against the wall at a steep angle, so casing wall loss is usually broad and scalloped and progresses from the inside outward. Casing failures are accelerated by operation far from the best efficiency point, which increases recirculation and the tip-speed-to-velocity mismatch. Where suction recirculation is present, the suction side of the casing and the impeller eye erode first, and no alloy upgrade will compensate for a pump running off its curve.

Impellers fail according to vane geometry. The leading edge of each vane, the vane-to-shroud fillet, and the shroud outside diameter see the highest local velocities because they carry the largest radius and therefore the highest tangential speed. Closed impellers typically lose metal at the vane eye and along the shroud periphery; open or semi-open impellers lose metal on the vane tips and on the back shroud. Hard-facing the vane leading edges and shrouds is the standard countermeasure, and it is effective precisely because the overlay's higher hardness addresses the mechanical term that dominates at these locations.

Hydrocyclones present the highest-velocity, highest-angle regime in most mining circuits. Feed enters tangentially at high speed, so the entry chamber, the upper cone and the apex or spigot region see severe impingement, and the apex wears fastest of all because the solids accelerate as the cross-section contracts. Wear is typically concentrated in a band aligned with the inlet and on the underside of the cone near the apex. Because the geometry itself is what accelerates the solids, prevention here is largely a surface-engineering problem: rubber or ceramic liners where the process allows, or an overlay on the metal surfaces where liners cannot be used. The substrate can then be a lower-cost alloy because its job is mechanical support rather than corrosion resistance, provided the chemistry is compatible with both the slurry and the overlay.

Let-down valves and control valves fail through a combination of flashing, cavitation and solids impingement, all acting on the trim. The seat, plug and downstream body are the critical zones; the failure pattern often shows a directional gouge from the seat into the body, sometimes with evidence of cavitation damage as clustered pitting on the downstream side. In acid service the trim also sees accelerated corrosion because the depressurisation step flashes dissolved gas and locally concentrates the liquor. The mitigation set is well established: use hardened trim or a solid tungsten carbide or ceramic trim where solids are coarse, stage the pressure drop so no single stage flashes, keep the valve out of the impingement path by using an angle body where geometry allows, and specify the body in a grade matched to the flashed liquor chemistry rather than the inlet chemistry.

Equipment and zone	Dominant mechanism	Typical local velocity	Standard for materials of construction	Mitigation
Pump casing cutwater	Steep-angle impingement plus vortex	4-8 m/s	ASTM B575, ASTM B463	Overlay, casing liner, restore BEP operation
Pump impeller vane leading edge	High-angle impingement	8-15 m/s	ASTM B575, ASTM B463, ASTM A240	Vane hard-facing, geometry change
Impeller shroud periphery	High-angle impingement	10-18 m/s	ASTM B575	Overlay, thicker shroud section
Hydrocyclone inlet chamber and apex	Severe high-angle impingement	10-20 m/s	ASTM G73 test basis	Ceramic or elastomer liner, overlay
Let-down valve seat and plug	Impingement plus flashing	20 m/s and above locally	NACE MR0175 / ISO 15156 where sour	Hardened trim, staged pressure drop
Pipe elbow extrados	Shallow-angle cutting	3-6 m/s	ASTM G119	Long-radius elbow, overlay, thicker wall
Absorber agitator blade	Shallow-angle cutting plus chloride pitting	2-5 m/s at tip	ASTM G48	Lower tip speed, super austenitic or C-276

Diagnosing which of these zones is failing, and why, is what turns a repeat failure into a design change. Photographs and dimensional traces sent with a returned part make that assessment much faster; the process used by the Shanghai failure analysis service is to compare the worn profile with the predicted flow field before and after any repair.

Weld Overlay, Hard-Facing and Surface Engineering for High-Wear Zones

Weld overlay and hard-facing work because they change the property that is actually limiting life at the worn zone, namely near-surface hardness, without forcing the whole component into an expensive alloy. The overlay resists the mechanical term while the substrate continues to provide corrosion resistance and structural strength.

The selection logic is to match the overlay to the dominant mechanism. Where the damage is shallow-angle cutting, a cobalt-based or nickel-based hard-facing deposit with a high volume fraction of hard carbides gives the best life. Where the damage is steep-angle impingement, a nickel-based alloy deposit with good toughness is preferable, because very hard but brittle deposits spall under repeated normal impacts and can fail faster than the base metal. Where the liquid is strongly acidic or chloride-rich, the overlay must itself be corrosion resistant, which generally points to nickel-chromium-molybdenum filler metals such as a C-276 or 625 type rather than a purely wear-resistant cobalt alloy.

Application technique is as important as deposit chemistry. Dilution from the substrate changes the deposit composition, so the first layer over a 316L or carbon steel substrate may be significantly lower in alloy content than the filler specification suggests. Overlay qualification per ASME Section IX with a measured dilution and a deposit chemistry check is the reliable route. Heat input must be controlled to avoid sensitising the substrate or creating a brittle HAZ, and interpass temperature limits are usually specified. For critical chloride service, an overlay should be verified with ASTM G48 pitting testing on a procedure qualification coupon, not only by hardness.

Option	Standard / filler reference	Typical hardness	Best for	Limitations
Nickel-based C-276 overlay	ASTM B575 filler chemistry, ASME IX procedure	20-30 HRC	Acid and chloride slurries with impingement	Lower hardness, dilution sensitive
Nickel-based 625 overlay	ASTM B443 filler chemistry, ASME IX	25-35 HRC	Mixed acid and chloride, good toughness	Moderate wear resistance
Cobalt-based hard-facing	Hard-facing filler by ASME IX qualification	40-55 HRC	Shallow-angle cutting, pump shrouds	Poor corrosion in strong acids
Nickel-based WC composite	Composite overlay by procedure qualification	55-65 HRC	Severe high-angle impingement	Spalling risk, difficult machining
Ceramic or elastomer liner	Liner system tested to ASTM G73 / G32	Not applicable	Hydrocyclones, coarse sand	Geometry limits, temperature limits
Nitriding or surface hardening	Controlled atmosphere per specification	Surface case only	Small trim parts, valve internals	Thin case, unsuitable for strong acid

Once the mechanism has been identified, the repair decision becomes a short matrix. The table below links the diagnostic signature to the most common root cause and to the intervention that actually stops the recurrence.

Diagnosis signature	Likely mechanism	Root cause	Primary corrective action	Supporting standard
Shallow flow-aligned grooves with comet-tail pits	Low-angle erosion-corrosion synergy	Velocity above threshold with fine hard solids	Reduce velocity, long-radius geometry, overlay	ASTM G119, ASTM G73
Broad scalloped loss at cutwater or apex	High-angle impingement	Vortex or jet impingement at steep angle	Overlay or liner, geometry change	ASTM G73, ASTM G40
Bright freshly exposed metal inside grooves	Passive film destroyed faster than repair	Solids loading above design, low pH	Alloy upgrade plus velocity reduction	ASTM G48, ASTM G28
Deep isolated pits with undercut edges	Local corrosion dominating	Chloride pitting or crevice under deposit	Deposit control, super austenitic or C-276	ASTM G48, NACE MR0175
Loss concentrated at weld cone or HAZ	Composition or condition problem	Dilution, sensitisation, poor procedure	Re-qualify welding, solution anneal	ASTM A262, ASTM G28, EN 10204
Thin, uniform wall loss on straight runs	Flow-accelerated corrosion	Velocity with low solids, temperature	Velocity control, thicker wall	ASTM G119
Spalling or cracking of overlay	Overlay too brittle for the angle	Wrong deposit hardness or toughness	Change overlay type, verify procedure	ASME IX, ASTM G65 style wear tests

Preventing Recurrence: Specifying, Inspecting and Monitoring Slurry Components

Prevention is a specification discipline rather than a single material choice, because the same alloy succeeds or fails depending on how velocity, geometry, welding and inspection are written into the purchase documents. A specification that names only the alloy and leaves condition, heat treatment and weld qualification open will eventually produce a failure that looks like a material problem but is not.

Begin with the hydraulic envelope. Define maximum and minimum flow, solids concentration and particle-size distribution, temperature, pH, chloride and fluoride levels, and the dissolved oxygen regime. Then set a velocity limit with margin below the alloy's threshold for that slurry, and enforce it in the piping layout. Document impingement-prone locations on the isometric, because those are the positions that need thicker wall, an overlay or an inspection point. Where a pump must operate over a wide range, note that off-design operation is itself a corrosion-engineering variable, since recirculation raises local velocities and impingement angles.

Write the material condition explicitly. Specify solution annealed condition, reference the applicable product standard, require EN 10204 3.1 certification as a minimum for critical items, and where the service is sour, require compliance with NACE MR0175 / ISO 15156. For welded fabrication, require procedure qualification per ASME Section IX with a deposit chemistry check on overlay work and a sensitisation test where intergranular attack is credible. Require hardness checks on hard-faced zones and dimensional verification of profile after overlay, since an overlay that changes the hydraulic profile can increase local velocity and create a new wear point.

Finally, monitor rather than wait. Establish baseline wall thickness by ultrasonic or radiographic survey at commissioning, repeat at intervals, and plot the loss rate against the design allowance so that a trend is visible before the wall reaches minimum. Where the loss rate is accelerating, the diagnosis should be re-run rather than the spare part simply replaced, because a trending change usually indicates a change in the process, the solids or the operating point. Sample the slurry at the same time as the thickness survey so that chemistry and solids data are contemporaneous with the metal loss. Interpreting the trend against grade limits is exactly what the engineering and quality team does when reviewing survey data for plant owners, and it is also the basis of the technical notes library kept for that purpose.

The table below cross-references the main alloy families across five specification systems, so that a specification written in one system can be translated into another without losing the essential requirements.

Alloy family (UNS)	US - ASTM / ASME	GB - GB/T	JIS	DIN / EN (W.Nr)	GOST
Ni-Mo-Cr, C-276 (N10276)	ASTM B575, ASTM B619, ASME SB575	GB/T 15007 (NS3304), GB/T 14992	JIS G 4901 to 4904 (NW0276)	DIN 17744, EN 2.4819	GOST 5632 (by chemistry)
Ni-Cr-Fe-Mo-Cu, Alloy 20 (N08020)	ASTM B463, ASTM B729, ASTM B906	GB/T 15007 (NS1402 family)	JIS G 4901 to 4904 (by chemistry)	DIN 17744, EN 2.4660	GOST 5632 (by chemistry)
Ni-Fe-Cr, 825 (N08825)	ASTM B424, ASTM B906	GB/T 15007, GB/T 14992	JIS G 4901 to 4904 (NCF 825)	DIN 17744, EN 2.4858	GOST 5632 (by chemistry)
Ni-Cr-Mo, 625 (N06625)	ASTM B443, ASTM B446, ASTM B906	GB/T 14992 (NS4301 family), GB/T 15007	JIS G 4901 to 4904 (NW6625)	DIN 17744, EN 2.4856	GOST 5632 (by chemistry)
Super austenitic 6Mo (S31254 / N08367)	ASTM A240, ASTM A312, ASTM B688	GB/T 20878, GB/T 24511	JIS G 4304 / G 4305	EN 1.4547, EN 1.4529	GOST 5632 (by chemistry)
Austenitic 316L (S31603)	ASTM A240, ASTM A312, ASTM A480	GB/T 20878, GB/T 24511, GB/T 14976	JIS G 4304 / G 4305	EN 1.4404, EN 1.4435	GOST 5632 (08Kh17N13M2)
Commercially pure nickel (N02200 / N02201)	ASTM B160, ASTM B162, ASTM B906	GB/T 2054, GB/T 15007	JIS G 4901 to 4904	DIN 17740, EN 2.4060 / 2.4066	GOST 492

Product availability in the forms above lets a designer specify bar, plate, seamless tube or forged trim in the same grade family, which keeps weld count low and removes fabrication-induced weak points. Where a component can be machined from solid rather than fabricated from plate, the erosion-corrosion performance improves simply because the impingement zone no longer contains a weld.

Standard Index

Standard	Title/scope	Covers	Form
ASTM B575	Nickel-chromium-molybdenum alloy plate, sheet and strip	C-276 composition, anneal condition, mechanicals	Plate, sheet, strip, bar
ASTM B463	Nickel-iron-chromium-molybdenum-copper alloy plate, sheet and strip	Alloy 20 composition and properties	Plate, sheet, strip
ASTM B729	Nickel-iron-chromium-molybdenum-copper alloy seamless pipe and tube	Alloy 20 pipe and tube	Seamless pipe, tube
ASTM B424	Nickel-iron-chromium-molybdenum-copper alloy plate, sheet and strip	825 composition and properties	Plate, sheet, strip
ASTM B443	Nickel-chromium-molybdenum-columbium alloy plate, sheet and strip	625 composition and properties	Plate, sheet, strip
ASTM B906	General requirements for flat-rolled nickel and nickel alloy plate, sheet and strip	Tolerances, general delivery requirements	Plate, sheet, strip
ASTM A240 / A480	Chromium and chromium-nickel stainless steel plate, sheet and strip; general requirements	316L and super austenitic flat product	Plate, sheet, strip
ASTM E8 / E21	Tension testing of metallic materials, ambient and elevated temperature	Tensile, yield, elongation basis	All wrought forms
ASTM G32	Cavitation erosion using vibratory apparatus	Cavitation erosion resistance	Laboratory specimens
ASTM G73	Liquid impingement erosion testing	Impingement erosion resistance and rate	Laboratory specimens
ASTM G40	Terminology relating to wear and erosion	Surface descriptor definitions	Reference terminology
ASTM G119	Determining synergism between wear and corrosion	Synergy ratio calculation	Laboratory specimens
ASTM G48	Pitting and crevice corrosion resistance of stainless steels and related alloys	Critical pitting temperature, chloride service	Coupons, qualification
ASTM A262 / G28	Detecting susceptibility to intergranular attack; sensitisation in wrought nickel-rich alloys	Heat treatment verification	Coupons
NACE MR0175 / ISO 15156	Materials for use in H2S-containing environments in oil and gas production	Sour service acceptability	All forms
EN 10204	Metallic products - types of inspection documents	3.1 and 3.2 certification	All forms
ASME Section IX	Welding, brazing and fusing qualifications	Overlay and weld procedure qualification	Fabricated assemblies
GB/T 14992 / GB/T 15007	Chinese superalloy designations; corrosion-resistant alloy grades	Chinese grade equivalents	Bar, plate, tube
GB/T 20878 / GB/T 24511	Stainless steel grades; stainless plate for pressure equipment	Chinese stainless designation and plate	Plate, sheet
JIS G 4901 to G 4904	Corrosion-resistant and heat-resistant nickel alloy bar, plate, seamless tube and welded tube	Japanese nickel alloy product standards	Bar, plate, tube, pipe
DIN 17744 / EN 10095 series	Wrought nickel alloys; heat-resisting materials	German and European grade and W.Nr systems	All wrought forms
GOST 5632 / GOST 492	Corrosion-resistant, heat-resistant steels and alloys; nickel	Russian grade system	All wrought forms

FAQ

Q1: What exactly is erosion-corrosion synergy and how is it measured?

Erosion-corrosion synergy is the extra material loss produced when mechanical erosion and electrochemical corrosion act together, above what the two would cause separately. The mechanical action strips the passive film and removes soft or partially dissolved metal; the freshly exposed surface then corrodes at a much higher rate than a filmed surface would, and the resulting roughening makes the next particle impact more damaging. ASTM G119 provides a framework to quantify it: measure the static corrosion rate, the erosion rate in an inert slurry, and the combined rate in the real environment, then calculate a synergy ratio. A ratio above roughly 1.5 to 2 indicates the interaction is the dominant mechanism. In practice most plant owners do not run the full programme, but even a simplified set of three tests on the candidate alloy and the actual slurry will separate a chemistry problem from a hydraulics problem — which is the decision that matters when choosing between an alloy upgrade and a design change.

Q2: Why does 316L fail quickly in slurries where C-276 lasts for years?

The difference comes down to passive film stability and repassivation speed after an impact, not to bulk hardness. C-276 contains roughly 15 to 17% molybdenum, which strongly promotes repassivation in reducing acids and chloride-bearing liquors, so a gouge begins to re-film immediately after the particle leaves. Type 316L contains only 2 to 3% molybdenum.

In a slurry with chloride and acid, 316L's film reforms slowly and is locally unstable, so the corrosion half of the synergy term grows quickly once erosion has begun. Hardness values are closer than most people assume: annealed C-276 typically sits around 185 to 200 HB and 316L around 130 to 180 HB, so neither is a wear material. Plant experience therefore shows 316L thinning and pitting in a matter of months in aggressive slurry while C-276 survives years, at a much higher material cost. Hangbo Alloy routinely sees this pattern in returned pump trim, where the 316L part shows grooving plus pitting and the C-276 equivalent shows only shallow polishing.

Q3: Where does Alloy 20 beat both 316L and C-276 in slurry service?

Alloy 20 is usually the best economic choice in sulphuric and phosphoric acid slurries that also contain chlorides, because copper and molybdenum together give it resistance to acid attack that 316L cannot match, while its nickel content keeps it ductile and weldable. It performs particularly well in wet-process phosphoric acid circuits, in acid slurry piping and in agitator shafts where the chemistry is the limiting factor and the solids are moderate. C-276 is more corrosion resistant in strongly reducing or high-chloride conditions, but it costs considerably more and its additional molybdenum is wasted where the acid is already within the Alloy 20 envelope. The practical rule is to specify Alloy 20 where the acid is the problem and solids are secondary, and to specify C-276 where chlorides, fluorides or reducing conditions also drive attack. Where both chemistry and severe impingement are present, the most economical answer is often Alloy 20 with a hard-facing overlay at the impingement zones rather than a solid C-276 component.

Q4: What velocity limit should be written into a slurry piping specification?

There is no single published limit that applies to all nickel alloys, and any specification should state that the value is a design guideline to be confirmed against the actual slurry. As a starting point, high-solids acid slurries are usually limited to about 3 to 5 m/s, low-solids neutral liquors can tolerate more, and any circuit containing hard particles above roughly 200 to 300 micrometres should be treated as an erosion-dominated environment above about 3 m/s. Chloride and low pH shift the limit downward because film repair slows. The limit should be applied to the local velocity at the wall, not the bulk line velocity, and it should be applied separately at elbows, tees, reducers and valve downstreams where the local vector is higher or steeper. Where a plant already has trend data, the observed loss rate should override generic guidance — a measured rate from the actual circuit is far more reliable than a table value, and it is the number a failure analysis report should quote.

Q5: How do sand concentration and particle size change the failure mode?

Both increase the mechanical term, but they do so in different ways. Raising the solids concentration from about 1% to about 5% by weight typically increases the number of impacts per unit area and disturbs the flow near each impacted zone, so the measured loss can more than triple rather than rising proportionally. Particle size changes the momentum carried by each impact, and momentum is what does the damage, so a small population of coarse hard particles can cause more loss than a much larger population of fines. Above roughly 200 to 300 micrometres in hard mineral slurries such as silica sand, impingement becomes the dominant mechanism in most nickel alloys once velocity exceeds about 3 m/s. The practical consequence is that a specification based on weight percent alone is not enough: it should also state a maximum particle size and, where possible, a hardness, because a soft coarse solid behaves quite differently from a hard one of the same size.

Q6: Why does impingement angle matter more than velocity in some failures?

Angle determines which mechanism removes the metal, and the material response to the two mechanisms is not the same. At shallow angles, typically below about 20 degrees from the surface, ductile metals lose material by cutting and ploughing, producing flow-aligned grooves. At steep to normal angles, typically 60 to 90 degrees, the repeated impact causes plastic deformation, cracking and detachment, and brittle or heavily cold-worked surfaces suffer much more. Many nickel alloys are ductile in the annealed condition, so they resist glancing flow reasonably well but are vulnerable to a direct jet. This explains why valve seats, orifice plates, pump cutwaters, hydrocyclone apexes and elbow extrados fail first: the local geometry converts a moderate bulk flow into a near-normal high-velocity jet at the wall. Angle also explains why the same alloy performs well in a straight pipe and badly a metre downstream of a partly open valve, where the jet angle is steep and the local velocity is multiplied by the contraction.

Q7: When is a weld overlay better than simply upgrading the whole component?

Overlay wins whenever the failure is local rather than general, which is the usual case in slurry service. The impingement zones are a small fraction of the component surface: a pump cutwater, a vane leading edge, a valve seat, a hydrocyclone apex. Overlaying those zones with a nickel-based or cobalt-based deposit addresses the mechanical term directly and leaves the rest of the part in a corrosion-resistant alloy that does not need to be hard. This is normally far cheaper than making the entire part from a higher-alloy material and often gives better life, because the overlay can be selected for wear while the substrate is selected for chemistry. The approach requires discipline, however: dilution from the substrate changes the deposit composition, so the first layer may not match the filler specification, and heat input must be controlled to avoid sensitising the base metal. Qualify the procedure under ASME Section IX with a deposit chemistry check and, in chloride service, a pitting test on a qualification coupon.

Q8: How do I distinguish an erosion-corrosion failure from pure corrosion on a returned part?

Look at the geometry and the distribution of loss before doing any metallography. Pure corrosion tends to be generalised or localised in patches unrelated to flow direction, with dark, tightly adherent corrosion product, undercut pit edges and no directional pattern. Erosion-corrosion shows a directional signature: grooves aligned with the local flow, sand-polished facets, comet-tail pits trailing behind each groove and a crescent-shaped worn zone on the downstream side of a weld or step. Bright, freshly exposed metal inside the damaged area is a strong sign that the film was being removed faster than it could reform. Location is equally diagnostic: impingement damage concentrates at the outer radius of a bend, at a valve seat, at a weld root protrusion or at a pump cutwater, whereas pure corrosion can appear anywhere including low-flow areas and crevices. Finally, cross-check with process data — if the failure coincides with a change in solids content, velocity, pH or chloride level, the synergy explanation becomes the working hypothesis.

Q9: What data should accompany a returned part for a proper failure analysis?

Send the part itself, or a cut section through the worst worn zone, together with photographs of the part in situ before removal so the flow direction can be established. Include the process data sheet: fluid composition, pH, temperature, chloride and fluoride levels, dissolved oxygen, solids concentration by weight, particle-size distribution and the design and actual flow rates. Add the operating history: hours run, any off-design operation, the date of the last thickness survey, and the wall thickness at that time. Include the material certification, the weld procedure and any overlay qualification records, because composition and condition are often part of the cause. Finally, state what the component is supposed to do hydraulically, so the analyst can compare the measured loss location with the predicted local velocity and impingement angle. With that package, the diagnosis usually takes days rather than weeks, and the corrective action can be tested against the real flow field.

Q10: How often should wall thickness be surveyed in slurry service?

Survey at commissioning to establish a baseline, then at intervals set by the predicted loss rate: if the design allowance permits, say, 3 mm of loss over 10 years, survey at six-month intervals in the first two years so that the real rate is measured early, then extend the interval once the trend is known. Always survey the same fixed locations, and mark them permanently so that successive readings are comparable. Include the impingement-prone positions identified during design, not only straight runs, because those locations carry most of the risk. Where a measured rate exceeds roughly half the design allowance per year, treat it as an accelerating problem and re-run the diagnosis rather than simply scheduling an earlier replacement. Pair every survey with a slurry sample taken at the same time, so that chemistry and solids data are contemporaneous with the metal loss. Trending against contemporaneous process data is what turns a thickness survey into a predictive tool rather than a record-keeping exercise.

Q11: Are there cheaper alternatives to a solid C-276 component?

Yes, in many cases, provided the failure is mechanical rather than chemical. Where the slurry chemistry is already within the envelope of Alloy 20, a super austenitic or even a 316L substrate, the remaining problem is surface wear, and that can be addressed with an overlay, a hard-facing deposit, a ceramic or elastomer liner, or a thicker section at the impingement zone. A hydrocyclone with a 316L or Alloy 20 body and a ceramic apex liner often outperforms a solid C-276 cone at a fraction of the cost. The prerequisite is a correct diagnosis: if the chemistry is actually outside the substrate's envelope, an overlay will fail from underneath by corrosion and the saving will be lost. This is why the diagnosis step comes first and why the corrective action should always be checked against both the erosion and the corrosion term, separately, before cost is compared.

Conclusion: Diagnosing Erosion-Corrosion Before You Buy the Replacement

Erosion-corrosion failure analysis is a sequence, not a guess: read the geometry and flow direction on the returned part, separate the erosion term from the corrosion term, compare the measured local velocity and solids loading against the alloy's threshold, and only then choose between an alloy upgrade, a geometry change and a surface treatment. C-276 and Alloy 20 outperform 316L in slurry service for specific compositional reasons, but neither will survive a circuit that runs above its velocity limit or a weld procedure that leaves the heat-affected zone sensitised. Prevention is therefore written into the specification — velocity limits, solids and particle-size data, solution annealed condition, qualified overlay procedures, EN 10204 documentation and a thickness survey programme tied to contemporaneous slurry sampling. Get the diagnosis right and the correct fix is usually obvious, and usually cheaper than the repeat failure.

If you have a failed impeller, valve trim, elbow or hydrocyclone cone in hand, send the photographs, the wear profile and the process data, and Shanghai Hangbo Alloy Group will match the diagnosis to the right grade, form and surface treatment before you order a replacement.

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

Email: sales@hangboalloy.com | WhatsApp (Lisa): +86 13611656360 | www.nickel-alloy.com

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Ni: NI200-N6 | Monel: 400-502, K-500, R-405 | Inconel: 600-740H, X-750, 725 | Incoloy: 800-27-7MO | Hastelloy: C-276-HYBRID-BC1, N | Super Austenitic: 904L-AL-6XN | Chromium-Rich: Alloy 33 | High-Strength Stainless: Nitronic 50/60, PH13-8Mo | Chlorination: Alloy 45 | Nimonic: 75-263 | Haynes: 25-HR-160 | RA: 330/333 | Precision: 1J22-4J45 | High-Strength: 18Ni250-AerMet100 | Heating: Cr20Ni80/60 | Specialty: Alloy 20-602CA

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