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Views: 0 Author: Site Editor Publish Time: 2026-11-11 Origin: Site
Pickling and passivation address different surface problems on stainless steel flanges. One batch may arrive with dark scale from heat treatment. Another may look clean but carry iron transferred during machining or handling. Ordering both as “pickling passivation” leaves the supplier to guess what the finished surface must meet.
Start with the condition of the surface and the stage at which you need to accept it. That helps you specify the right work, protect the gasket face and request evidence that matches the order.
Pickling removes heat scale and the affected surface layer beneath it. Consider it when the specification calls for removing scale or weld heat tint.
Passivation removes surface contamination such as free iron from suitably cleaned stainless steel and promotes formation of the passive film.
Clean first if oil or grease is present. Neither treatment is a substitute for degreasing.
Specify the finished condition. Identify which surfaces to treat, which dimensions and finishes to preserve, and how you will verify the result.
Question | Pickling | Passivation |
|---|---|---|
What problem does it address? | Oxide scale and the chromium-depleted surface beneath heat tint | Free iron and other surface contamination that the specified treatment addresses |
What happens to the surface? | The process removes a thin layer of metal along with the scale | The treatment promotes a passive film on a chemically clean surface |
What might you see? | The part may look duller after treatment | The appearance may change very little |
What should the order clarify? | Areas to descale, surfaces to protect and final finish requirements | Treatment specification, applicable verification method and records |
For a stainless steel flange, specify the operation in terms of the surface problem you need to solve. A general finish label does not define that problem or the acceptance criteria.
Heating during welding or heat treatment can produce a coloured oxide layer. The stainless steel immediately beneath it may have less chromium than the unaffected material. Removing only the visible colour does not establish that the affected layer has been addressed.
When the project requires heat tint removal, identify the weld and adjacent areas that need treatment. A colour photograph can document their location. It cannot measure how much metal a treatment removed or demonstrate compliance on its own.
Free iron here means iron contamination on the surface. It does not mean the iron that is part of the stainless steel alloy. Shared tooling, steel particles and contact with carbon steel can transfer contamination to a machined part.
A flange can look bright and still carry this contamination. If the order calls for passivation, use the agreed verification method alongside visual inspection. The absence of a visible stain is not a substitute for that check.
If scale remains on a flange body, ask how the supplier will remove it and what surface condition will follow. Passivation alone should not be specified as the scale-removal step.
Distinguish scale on the body from a mark on the gasket face. A scratch across the sealing area needs its own assessment. Surface treatment cannot replace a decision about whether that mark meets the drawing.
A machined flange may need cleaning and specified passivation even when it looks uniform. In this case, the objective may be to control transferred iron and verify the final condition.
Check for machining fluids or protective oils first. The treatment specification should address precleaning and the finished surface. There is no reason to add pickling automatically if scale removal is not part of the requirement.
A weld neck flange can arrive with a treated surface and then develop new heat tint at the pipe weld. Its delivery records cover the flange before that welding took place.
If the completed spool has a surface requirement, assign responsibility for it in the fabrication scope. Identify which surfaces the fabricator must assess after welding. This avoids a gap between the supplier's inspection and the condition of the installed assembly.
The flange body, bore, threads and gasket contact surface may have different requirements. Mark the relevant areas on the drawing. State where treatment is required and where the process must preserve the specified finish or dimensions.
Pickling removes metal, so consider its effect on the final surface. If the drawing specifies serrations or a roughness range on the gasket face, check that requirement in the condition in which you will accept the part.
A dull body can be an expected result of pickling. It says little about whether the gasket face meets its finish requirement. Likewise, a polished face does not demonstrate that the part passed a chemical treatment check.
Confirm whether the required condition applies after final machining, after welding or at delivery. A measurement or treatment report from an earlier stage may no longer describe an area that someone later machined or welded.
Discuss cleaning, rinsing and protection through shipment in the order. Pay particular attention to features that can retain residue, such as recesses or threaded holes. Handling and packaging should also preserve the surface condition you are paying to verify.
ASTM A380/A380M gives guidance on cleaning, descaling, pickling and passivation of stainless steel parts, equipment and systems. It is useful when defining how the project will address contamination or oxide scale.
ASTM A967/A967M covers chemical passivation treatments for stainless steel parts. It includes alternative tests with acceptance criteria for checking treatment effectiveness. Agree on the applicable treatment and verification requirements for the order. The standard number alone does not tell the inspector which result to expect.
Visible condition: Use the agreed visual criteria for scale, heat tint or other surface findings. Photos can record the area and condition checked.
Passivation verification: Record the selected test, the lot or parts it covers, and the result where the order requires that test.
Machined finish: Record the gasket-face dimensions or roughness separately if the drawing requires them.
These checks answer different questions. A treatment record does not establish that a particular alloy is suitable for the fluid or temperature in service. Review the operating conditions separately when assessing flange corrosion risks.
Start with the material grade and drawing revision. Describe the surface as it will arrive at the treatment stage, especially if it has scale, machining residue or a completed weld. Then specify the work and the final condition needed.
A useful enquiry covers five points:
Objective: Remove specified scale or heat tint, provide chemical passivation, or meet both objectives where required.
Coverage: Identify the flange body, bore, threads, gasket face and any welded areas that need attention.
Timing: State whether the requirement applies after machining, after spool welding or at another agreed stage.
Limits: Carry over the drawing's sealing-face finish and dimensional requirements.
Evidence: State the applicable standard and edition, verification method, acceptance criteria and records.
For a machined flange without a scale-removal requirement, an enquiry could say:
Chemical passivation after final machining and cleaning, to ASTM A967/A967M, edition as specified in the purchase order. Agree the treatment and verification method before production. Preserve the gasket-face finish shown on the drawing and provide the agreed lot record.
For a welded spool with heat tint, describe the weld areas and the required final condition instead of copying that wording unchanged. Agree on the surface treatment and acceptance of the completed spool.
Specify pickling when the job calls for removing scale and the affected surface beneath it. Specify passivation when the order requires a chemically treated, verified surface on a suitably cleaned part. The drawing and acceptance plan should carry those requirements through the last machining or welding stage that affects the surface.
SDFL supplies stainless steel flanges for industrial piping. Share the flange drawing and the expected surface condition so the treatment scope and inspection requirements can be clarified before production.
Need to specify the finish for a flange order?
A separate passivation step depends on the purchase specification, treatment route and acceptance criteria. If both operations are required, state them separately. Also specify how the final condition will be verified.
Conventional passivation is not the operation to rely on for removing heat scale or the affected layer beneath it. If the project calls for heat tint removal, specify an appropriate descaling or surface restoration process.
It can reduce the brightness of the surface. Agree on any appearance requirement in advance, particularly if exposed and hidden areas need to match. Check the gasket face against its own drawing requirements.
A material certificate can support grade and material traceability. For passivation, request records that identify the treatment, the covered lot and the agreed verification result. Review what each document actually certifies.
Electropolishing removes surface metal using an electrochemical process and can produce a smoother, brighter finish. A drawing that calls for electropolishing may have its own appearance and roughness requirements. Confirm those separately from any passivation requirement.
