Custom corrosion-resistant parts fabrication: choosing the material and the protection

Start from the service, not from the catalogue

Custom corrosion-resistant parts fabrication starts with the service conditions — fluid, chlorides, temperature, pressure, abrasion — and only then picks the material: austenitic or duplex stainless, nickel alloy, titanium, FRP or coated carbon steel. The protection is part of the design, and fabrication must not undo it.

Premature failures of custom parts usually trace back to one of three shortcuts: the material was copied from the old part without asking why it failed; the alloy was right but welding and finishing destroyed its passive surface; or the part created a crevice or a galvanic couple with the equipment around it.

If the part must be rebuilt from a worn original with no drawings, the survey method is in reverse engineering a part with no drawings. This article covers what comes once the geometry is known: material, protection and fabrication.

The service conditions that decide the material

Before any grade is named, five questions need written answers:

• Fluid and chemistry. What touches the part: water, brine, acid, caustic, solvent, slurry? Concentration, pH, oxidizing or reducing character, and contaminants — chlorides above all, but also fluorides, sulfides and dissolved oxygen.
• Temperature. Normal, maximum and upset conditions. Pitting and stress corrosion risk on stainless rise with temperature; polymer and coating options are bounded by it.
• Pressure and mechanical load. Pressure, vibration, cyclic stress, bolt loads: they set wall thickness and rule out materials too weak at temperature.
• Abrasion and velocity. Suspended solids, impingement, cavitation can strip a passive film as fast as it forms.
• Wet/dry cycles and concentration points. Evaporation, splash zones, deposits and stagnant areas concentrate chlorides far beyond the bulk analysis.

Add what is already installed: the material of the adjacent piping, shell or housing matters as much as the fluid.

Material selection table: stainless, duplex, nickel alloys, FRP, coated steel

The table is qualitative on purpose. Limits depend on the combination of conditions, and the final choice is checked against corrosion data, the applicable code and experience in comparable service.

There is no "best" material: titanium, excellent in seawater, fails in fluoride media. And the cheapest material per kilogram is rarely the cheapest part — a 316L part that pits through early costs more over its life than the alloy that would have lasted.

PREN and chlorides: screening stainless grades

Chlorides are the most frequent reason a stainless part fails early. The first screening tool is the pitting resistance equivalent number: PREN = %Cr + 3.3 %Mo + 16 %N. Values rise from 304L to 316L, then to duplex 2205, then to super duplex and 6Mo grades — which is why the molybdenum and nitrogen content has to be specified, not just "stainless".

PREN describes the alloy, not the part: it ignores temperature, crevices, deposits, weld quality and surface condition. It shortlists; critical pitting temperature data (ASTM G48), service experience and design decide. The mechanism, the test methods and indicative values by grade are in pitting corrosion explained. Where it is critical, PREN is calculated from the actual heat's certificate.

When abrasion adds to corrosion: wear-resistant materials

Slurry pumps, chutes, agitators and nozzles see corrosion and wear together, and the combination is worse than either alone: abrasion strips the passive film or the coating, and corrosion attacks the fresh surface. The usual answer pairs a corrosion-resistant base with a wear layer — hardfacing, ceramic-filled epoxy, ceramic tiles — or uses abrasion-resistant steel where the medium allows. The trade-offs are compared in alumina ceramic vs AR steel. Stainless alone is a poor wear material; duplex is stronger than austenitic grades, but strength is not abrasion resistance.

Coated steel or alloy: choosing the metal protection solution

A coated or lined carbon steel part is a legitimate engineering choice, not a cheap substitute. It tends to win when:

• the part is large or heavy — housings, water boxes, tank internals, ducts — and solid alloy would be disproportionate;
• the service is within the temperature and chemical resistance of a proven coating or lining system;
• the protected surfaces are accessible for inspection (dry film thickness, holiday testing) and touch-up;
• abrasion dominates, where a ceramic-filled coating outperforms any stainless grade;
• the alloy's lead time does not fit the shutdown window.

An alloy wins when the part has internal passages, threads or tight tolerances that cannot carry a film; when temperature or chemistry exceeds what coatings tolerate; when high velocity would erode a coating; or when the coated steel would be coupled to a much more noble metal, because every holiday then becomes a small anode facing a large cathode. Coating-friendly design — radiused edges, continuous welds, film thickness allowance on sealing faces — belongs on the fabrication drawing.

Fabrication: what welding and finishing do to corrosion resistance

A corrosion-resistant alloy delivers its resistance only if fabrication preserves it.

Welding custom stainless and duplex parts. Low-carbon "L" grades limit sensitization (chromium carbide precipitation in the heat-affected zone). Heat input and interpass temperature follow the qualified procedure, especially on duplex: too little heat leaves excess ferrite, too much promotes intermetallic phases, which ASTM A923 detects when specified. Roots are back-purged with inert gas to avoid oxidized, "sugared" roots. Procedures and welders are qualified to ASME Section IX or the governing code.

Pickling and passivation. Welding leaves heat tint, a chromium-depleted oxide where pitting starts preferentially. It is removed by pickling or mechanical means, followed by passivation to restore a clean, continuous passive film. ASTM A380 covers cleaning, descaling and passivation of stainless steel parts, equipment and systems; ASTM A967 specifies chemical passivation treatments and the tests that verify them. Iron embedded by carbon steel tools or grinding discs causes rust spots on new stainless parts: dedicated tools prevent it.

Surface finish. A smoother finish holds fewer deposits and initiation sites; a coarse, contaminated grind can perform worse than mill finish.

Designing out crevices. Many stainless failures start in a crevice the design created: lap joints, intermittent welds, back-to-back plates, gaskets overhanging the bore, threaded joints in chloride service, pockets that do not drain. Continuous full-penetration butt welds, drainable geometry and correctly sized, non-absorbent gaskets remove them. Why even high alloys fail in crevices is explained in crevice corrosion explained.

Galvanic compatibility with existing equipment. A stainless or nickel alloy part bolted into a carbon steel or cast iron system is a galvanic couple whenever an electrolyte is present. Distance in the galvanic series indicates the driving force; the area ratio decides how damaging it is, and a small anode against a large cathode corrodes fast. Remedies: closer materials, insulated joints (isolation gaskets, sleeves, washers) and, if only one side can be coated, coating the more noble side. See galvanic corrosion in the glossary.

Documentation and traceability

A corrosion-resistant part is only as reliable as the evidence that it is made of what was specified.

• Material certificates. An EN 10204 type 3.1 inspection certificate — issued by the manufacturer and validated by its authorized inspection representative, independent of production — or a mill test report giving chemical analysis and mechanical properties against the ASTM or ASME specification, tied to the heat number. Type 3.2 adds validation by the purchaser's representative or the inspector designated by regulations, where the code or the owner requires it.
• Traceability. Heat numbers transferred to cut pieces, so every component ties back to its certificate; positive material identification (PMI) where a mix-up would be critical.
• Welding records — procedure specifications and qualification records, welder qualifications.
• Finishing and inspection records — passivation and its verification, dimensional report, and for coated parts dry film thickness and holiday test results.

That file becomes the definition of the part: the next order is placed against it, not against a worn sample.

What drives the lead time

The drivers are known: availability of the grade in the required form and thickness (common 316L plate and bar are easier to source than nickel alloy forgings or titanium); castings and patterns; a welding procedure to qualify for an unusual alloy combination; and finishing, coating cure and inspection. Settling the material at quotation stage, with an acceptable alternative grade, is the best way to fit a shutdown window. When the question is whether a new part is justified at all, the criteria in repair or replace industrial equipment apply.

FAQ

Is 316L stainless enough for chloride service? It depends on chloride concentration, temperature, pH, oxygen and crevices. 316L is often adequate in cool, low-chloride water and often inadequate in warm brines, seawater or under deposits. Screen with PREN, then confirm with critical pitting temperature data and service experience.

Which material certificate should I ask for? At minimum, an EN 10204 3.1 certificate or a mill test report traceable to the heat number of each alloy component. A 3.2 certificate is specified when the code, the owner or the criticality of the service requires independent validation.

Do stainless parts have to be passivated after fabrication? Welded, ground or shop-handled stainless parts should have heat tint and embedded iron removed, then be passivated per ASTM A380 or A967 and verified. Skipping this step is a frequent cause of early rust staining and pitting at welds.

When is FRP a better choice than stainless? In many acid and chloride services at moderate temperature and pressure, where stainless would require a costly high alloy — within FRP's limits on temperature, solvents, impact and abrasion.

Can a stainless part be installed in a carbon steel line? Yes, if the galvanic couple is managed: isolation kits at flanged joints, attention to the area ratio, and coating of the more noble side where only one side can be coated.

In practice

Induscoat fabricates custom corrosion-resistant parts — material selection from the service conditions, stainless and alloy fabrication, pickling and passivation, coating or lining of carbon steel parts, and a documented inspection file — through its custom parts fabrication service for plants in Quebec and Ontario, alongside industrial corrosion protection. The service conditions, the adjacent equipment's material and a drawing or photo of the part are enough to start: request a quote. Technical data sheets for the Induscoat range are published on the brand site, induscoat.com.