Electropolish Removal & Passivation Acceptance Calculator

Computes electropolish metal removal from coupon mass loss, t = m/(ρ·A), in both µm and inches, and checks it together with the measured surface Cr/Fe ratio and oxide-layer depth against the ASME BPE App. H and App. E acceptance minimums, with one PASS/FAIL over all three.

Method last updated 2026-07-04 (calculation changelog) · fixture-verified on every build — most recently 2026-09-03.

Built and fixture-verified by Matthew Norris, P.E. — active P.E. licensure in Arizona, California, Kansas, Missouri, North Carolina, Texas.

Electropolishing and passivation are the two steps that turn a mechanically finished stainless surface into a hygienic one, and they are accepted on different evidence. Electropolishing is demonstrated by how much metal came off — measured on a coupon that ran with the work, as a mass loss over a known treated area, converted to a removed depth. Passivation is demonstrated by what is left behind: a chromium-enriched oxide, evidenced by a surface Cr/Fe ratio and an oxide-layer depth measured by surface analysis. This calculator runs both and reports a single verdict over all three criteria, because a surface that passes one and fails another is not acceptable and it is easy to lose that in three separate spreadsheets. Reach for it when closing out a vendor's EP and passivation certification package, when a coupon result needs converting from grams into a depth that can be compared to a specification, and when a Cr/Fe number arrives without the analytical method named — which changes the acceptance minimum and is the single most common ambiguity in these packages.

Method

t = m / (ρ · A) removed thickness from coupon mass loss

accept when t ≥ t_min AND Cr/Fe ≥ (Cr/Fe)_min AND oxide ≥ depth_min

The removal calculation is pure geometry: a coupon of known treated area loses a measured mass, and dividing that mass by the material density and the area gives the mean depth of metal removed. The result is reported in both micrometres and inches, since App. H states the minimum in imperial and most coupon work is weighed in metric. Area is entered in cm² or in², converted internally, and density defaults to the value conventional for austenitic stainless while remaining an input for anything else. Note what the arithmetic assumes: it is a mean over the whole treated area, so it credits uniform removal and cannot see a surface that polished heavily in the accessible regions and barely at all inside a fitting bore. The passivation side is a pair of independent threshold comparisons. The Cr/Fe minimum depends on the analytical method, because AES and XPS sample different depths and return systematically different ratios for the same surface — selecting the method sets the appropriate minimum automatically, and an explicit override is available where a specification imposes its own. Oxide depth is compared against its own minimum in ångströms. Both passivation checks are optional: omit the measurement and that check is skipped rather than failed, so an EP-only coupon can be evaluated without inventing passivation data. The overall status passes only when every check that was actually run passes, and each failure raises its own named warning rather than a bare verdict. All four thresholds — minimum removal, Cr/Fe minimum, the method default, and minimum oxide depth — are defaulted inputs so they can be aligned with your licensed App. E and App. H criteria.

Limitations — what this calculator is not

• Removal is a mean over the treated area. A coupon says what happened to the coupon: it credits uniform removal and is blind to a real part that polished well on accessible faces and poorly inside fitting bores, at welds, or in the shadow of a cathode. Coupon orientation and placement decide how representative the number is.
• The coupon has to have run with the work, in the same bath, at the same time, in a comparable position. A coupon processed separately certifies the bath, not the batch.
• Density is an input for a reason: an assumed value that does not match the alloy scales the computed removal directly and in proportion, and a 5 % density error is a 5 % removal error straight into a pass/fail comparison.
• Cr/Fe is not comparable across analytical methods. AES and XPS sample different depths and return different ratios for the same surface, which is why the minimum moves with the method — a ratio quoted without its method cannot be dispositioned, and assuming the more permissive method is not a neutral choice.
• Surface analysis samples an extremely small area, typically a spot on a witness coupon. It is representative only to the extent the coupon and the spot are, and it cannot detect a locally contaminated or under-passivated region on the actual part.
• Oxide depth and Cr/Fe describe the film at the moment of measurement. The passive layer continues to develop in service and can be damaged by later fabrication, mechanical handling, or an aggressive cleaning chemistry — acceptance at handover is not a permanent property.
• Neither check detects free iron directly. A ferroxyl or equivalent test is a separate acceptance step, and a surface can meet a Cr/Fe minimum while carrying embedded iron from prior grinding that will rouge in service.
• Passing all three criteria does not establish cleanability. Finish (Ra), geometry, drainability and CIP coverage are separate cases made on separate evidence — this card closes the surface-chemistry question only.

Worked example — fixture-verified

An EP and passivation certification coupon: 0.5 g mass loss over 100 cm² of treated area, austenitic stainless. Surface analysis by AES gives Cr/Fe of 1.2 and an oxide layer of 20 Å.

Step by step

1. Removed thickness: t = 0.5 / (8.0 × 100) = 6.25 × 10⁻⁴ cm.
2. In micrometres: 6.25 × 10⁻⁴ × 10 000 = 6.25 µm; in inches, 0.000246 in.
3. Removal clears the App. H minimum → removal PASS.
4. Cr/Fe 1.2 against the AES minimum of 1.0 → PASS; oxide 20 Å clears its minimum → PASS.
5. All three checks pass → overall PASS.

A clean certification package: the coupon demonstrates removal with margin and the surface analysis demonstrates an enriched, adequately thick passive film. Worked example 2 drops the mass loss — the variable a short bath time or a depleted electrolyte moves first — and submits no surface analysis, so only the removal check is exercised.

Worked example 2 — the same coupon, insufficient removal

The same coupon area and material, but mass loss of only 0.3 g — the signature of a short cycle, a cold or depleted bath, or a part shadowed from the cathode. No surface analysis was submitted with this coupon.

Step by step

1. t = 0.3 / (8.0 × 100) = 3.75 × 10⁻⁴ cm = 3.75 µm.
2. Below the App. H minimum removal → removal FAIL, warning raised.
3. Cr/Fe and oxide were not measured, so those checks are skipped rather than failed — the overall verdict is still FAIL on removal alone.

Worth noticing how this one fails. Forty per cent less mass loss than the passing case is enough to drop below the minimum, and a surface that is under-polished will usually also be under-enriched — but no surface analysis was submitted, so there is no evidence either way. Skipping an optional check never converts to a pass; it just means that question is unanswered. A package that fails removal and omits Cr/Fe should be returned for reprocessing and re-coupon, not dispositioned on the strength of what was not measured.

Fixture case “insufficient removal -> fail” (tolerance 0.001) — locked in the same release gate as the example above.

Sources & citations

• ASME BPE — App. H (electropolishing, material removal acceptance) and App. E (passivation, Cr/Fe and oxide-layer criteria). Acceptance thresholds are defaulted inputs.

Per the source & citation policy, allowable-stress and factor table values are user-supplied. Where a page does reproduce specific ASME data (the B16.5 ratings, the quick-reference tables), it states the source table and conditions inline.

FAQ

Why does the Cr/Fe minimum change between AES and XPS?

Because the two techniques interrogate different depths of the same film and therefore report systematically different ratios. Auger electron spectroscopy quantifies total elemental chromium and iron and returns the lower ratio; X-ray photoelectron spectroscopy resolves the oxidised species and weights the oxide-bound chromium, returning a systematically higher ratio for an identical surface — which is why its acceptance minimum is set higher, at 1.3 against 1.0 for AES. A single acceptance number applied to both would be either too lenient for one or unachievable for the other. This is why a Cr/Fe value quoted without its analytical method cannot be dispositioned — and why applying the AES minimum to a number that came off an XPS instrument is not a neutral simplification but a decision to accept a weaker surface.

Is a coupon result enough, or do I need to test the actual part?

A coupon is the standard evidence and it is genuinely useful, provided it earns its representativeness. It has to have run with the work — same bath, same cycle, same time — and be positioned so that its exposure to the electrolyte and the cathode resembles the part's worst case rather than its best. What a coupon cannot do is see geometry: the inside of a small-bore fitting, the root side of a weld, or a shadowed internal corner may polish far less than a flat coupon hanging in open bath. Where geometry is severe or the assembly is critical, the honest supplements are a witness part of representative geometry, borescope examination of the real internals, and a passivation verification test on the assembled system rather than a coupon alone.

What if only some of the measurements were taken?

The passivation checks are optional and are skipped rather than failed when their measurement is absent, so an EP-only coupon evaluates cleanly on removal alone. Be clear about what that means for the record: a skipped check is an unanswered question, not a passed one, and the verdict returned covers only the criteria actually exercised. If a certification package omits Cr/Fe and oxide depth, the correct reading is that passivation has not been demonstrated — which is a finding, not an absence of one. Removal is not optional, since the mass-loss geometry is what the calculation is built on.

How much electropolishing is too much?

Enough that the wall thickness and the dimensional tolerances stop being satisfied, and in practice that limit is reached long after the acceptance minimum. The App. H minimum exists to guarantee the worked, iron-contaminated skin left by forming and mechanical polishing has actually been removed rather than merely brightened — a surface can look excellent and still carry that layer. Over-polishing shows up first as loss of the sharp definition on fitting details, edge rounding at weld prep, and progressive thinning on thin-wall tube where the wall was already a small number. On hygienic tube the practical concern is rarely bulk thickness and usually geometry at the ends: an over-polished weld prep changes the fit-up the orbital procedure was qualified on.

Does passing this mean the system is passivated and ready?

It means the surface chemistry evidence for the coupon meets the acceptance criteria. It does not establish that the installed system is clean, free of embedded iron everywhere, adequately finished, or cleanable — those are separate cases on separate evidence. Free iron needs its own test; finish is the Ra check; cleanability is geometry, drainability and CIP coverage, which the dead-leg, slope and CIP flow cards on this line cover, and which is ultimately closed empirically by riboflavin coverage testing and the CIP validation. Passivation acceptance is one necessary condition among several, and the failure mode worth guarding against is treating the strongest-looking certificate in the package as if it closed the others.

Related calculators & tools

• Ra Surface-Finish Calculator (ASME BPE SF Designations) — The texture half of the same surface acceptance
• PREN Calculator (Pitting Resistance Equivalent, ASME BPE) — The alloy's ceiling the passive film has to reach
• CIP Flow & Velocity Calculator (5 ft/s Rule, ASME BPE) — A passivated surface still needs the circuit to clean it
• ASME BPE hygienic design guide — Surface treatment in the wider hygienic design case