Guide
What causes plating defects like blistering and peeling, and what adhesion test should the PO require?
Plating defects are flaws in an electroplated metal coating that make it fail its specification, and the ones that scrap whole lots are adhesion failures: blisters (domes where the coating has lifted), peeling or flaking (coating that comes away in sheets), and layers of a multilayer system separating from each other. Most of them start in surface preparation, before the first layer is deposited, and most can be caught by a simple qualitative adhesion test, provided the purchase order says which test and what counts as a pass.
By Saola Manufacturing engineering team · Published · Updated
This guide is written for the engineer or buyer holding a plated lot that blistered, peeled or flaked, at receiving inspection, after assembly or in the field. It ranks the causes, shows how to tell them apart from the failed parts themselves, gives a decision table for containment and corrective action, and lists the lines that belong on the drawing and purchase order so the next lot is tested to a written criterion instead of to someone’s thumbnail. It applies to zinc, nickel, copper-nickel-chromium and other electroplated coatings on steel, stainless steel, copper alloys, aluminum and zinc die castings.
On this page
How do you recognize a plating adhesion defect, and how is it different from other plating defects?
An adhesion defect is any place where the coating has separated, or can be made to separate, from the base metal or from the layer beneath it. Blisters, peeling, flaking and lifting at an edge are adhesion defects; pits, nodules, burning, roughness, stains and thin coating are workmanship or thickness defects with different causes.
The distinction matters because the two groups are handled differently. A thickness or appearance problem is usually a process-control issue on the plating line and is judged against the thickness and workmanship clauses of the coating specification. An adhesion problem means the bond itself is weak, so even parts that look perfect today may lift later in forming, crimping, heating, assembly or service. ASTM B571 states the principle plainly: in perfect adhesion the bond between coating and substrate is stronger than either material, good electroplating practice usually achieves it, and peeling, flaking or lifting under test shows that adhesion is less than perfect (ASTM B571-23).
Typical presentations at receiving inspection are:
- Blisters. Raised domes, sometimes only visible under a raking light, that crack or collapse when pressed. They may be present when the lot arrives or appear later, after a bake, a paint cure or a period in storage.
- Peeling or flaking. Coating that comes off in flakes when a part is bent, crimped, threaded, staked or scratched, or that lifts from sharp edges and holes.
- Interlayer separation. On a multilayer system such as copper plus nickel plus chromium, the top layer comes away and leaves an intact layer below it.
- Delayed blistering. Blisters that appear days or weeks after plating, often on castings, which point to something trapped under the coating rather than to a bad bath.
What causes blistering and peeling on plated parts, ranked by how often they occur?
The leading cause is a surface that was not fully clean and active when the first layer went on: residual oil, buffing compound, oxide, scale or smut. After that come substrates that need a special preparation route, porosity in castings, hydrogen in steel, and interruptions between layers.
Cleaning and activation. One plating troubleshooting guide attributes 90% or more of poor-adhesion problems on steel and cast iron to preparation before plating (Altmayer, Finishing and Coating, 2024). ASTM B322 opens with the point that the cleanliness needed for electroplating is greater than for most other finishes, and it describes a staged cycle of precleaning, alkaline cleaning and electrocleaning followed by an acid treatment that removes tarnish, oxide and fingerprints and activates the metal (ASTM B322-99(2025)). ASTM B183 sets out the same three stages for low-carbon steel: remove oil and soil, pickle off scale and oxide, then remove smut and activate the surface just before plating (ASTM B183-18(2022)). Skipping or exhausting any stage leaves a film the coating sits on rather than bonds to. The same troubleshooting guide adds two traps worth knowing: heat-treat scale that ordinary cleaning and pickling cannot remove, and acid dips contaminated with metals more noble than the base metal, which leave a loosely bonded immersion film under the plate. Upstream contributors on the part side include machining coolants and rust preventives that the cleaning line was not designed for, polishing and buffing compounds packed into recesses, heat-treat scale, and fingerprints from handling between machining and finishing.
Substrates that need a special route. Aluminum carries a tenacious oxide film and is normally plated over an immersion zinc or tin layer and a strike; ASTM B253 lists five such immersion and strike procedures for aluminum alloys (ASTM B253-11(2022)). Stainless steel carries a thin passive oxide film that has to be removed by activation just before plating; ASTM B254 warns that the film reforms if the parts dry or are exposed to oxygen-containing solutions, and the plate will then not adhere (ASTM B254-26). Zinc die castings for decorative plating are normally plated with copper first, then nickel and chromium (International Zinc Association). When a part made from one of these materials is run through a cycle meant for plain steel, adhesion fails across the whole lot.
Casting porosity and surface condition. On die castings, gas in pores just under the surface, or cleaning and plating solution drawn into surface-connected pores, can expand or react under the coating, which is why castings can blister after a bake or in storage. Laps and cold shuts that were not fully removed can also be lifted by the stress in the coating. Industry guidance for zinc die castings is that a casting to be electroplated must be substantially free of surface defects (International Zinc Association, polishing and plating). Over-polishing that cuts through the dense casting skin into porous metal below has the same effect.
Hydrogen in steel. Pickling and plating put hydrogen into steel. On high-strength parts the main concern is embrittlement, which ASTM B850 addresses with a post-plating bake that must come before any conversion coating (ASTM B850-98(2022)); hydrogen that collects at inclusions or laminations in the steel can also lift the coating as blisters.
Interlayer problems and coating stress. A nickel layer that is allowed to dry, sit or become passive before chromium is applied can shed the chromium, by the same mechanism B254 describes for stainless steel; a current interruption mid-cycle can leave a laminated coating; and a highly stressed coating can crack and lift on its own, especially at edges.
| What you see | Most likely cause | Quick check / first fix |
|---|---|---|
| Peeling across whole parts, flakes come off clean | Incomplete cleaning or activation; wrong cycle for the base metal | Examine flake underside for oil, smut or oxide; water-break test cleaned parts; strip, re-clean, re-activate, replate a sample |
| Whole lot of aluminum, stainless or zinc castings fails | Missing immersion layer or strike | Ask the plater which route was run; confirm against ASTM B253 (aluminum), ASTM B254 (stainless) or the route the spec requires |
| Blisters on castings, clustered at thick sections or after a bake | Surface-connected porosity trapping solution or gas | Section through a blister; review casting quality and polishing depth before replating |
| Blisters on high-strength steel found after the bake | Hydrogen at inclusions or laminations | Section the substrate; review pickling and bake records; check embrittlement risk |
| Top layer peels, layer below stays bonded | Passive or contaminated underlayer between tanks | Identify which interface failed; review transfer and reactivation between layers |
| Lifting or cracking at sharp edges only | High current density at edges, stressed coating, burrs | Check edge breaks and deburring on the drawing; review coating stress and racking |
| Blisters in blind holes, threads or seams | Trapped cleaning or plating solution, poor rinsing and drainage | Check hole design and drainage; ask for rinsing changes or plugging |
How do you diagnose which cause is behind a peeling or blistered lot?
Find the interface that failed, then look at the pattern. Where the coating separated (at the base metal, between layers, or inside a porous casting skin) and where on the part and in the lot the failures sit tell you more than any bath analysis.
- Quarantine and count. Segregate the lot, record how many parts are affected, and keep the packaging and lot labels. A defect on every part points to a process step that was missed; a defect on some parts points to the part or to racking.
- Open a blister or lift a flake. Look at both faces with a loupe or low-power microscope. Bare, bright base metal under a clean flake suggests the coating never bonded (cleaning or activation). A dull film, staining or residue suggests oil, smut or oxide. Liquid, white salts or corrosion product inside a blister suggests trapped solution. A coating that carries a thin layer of base metal with it suggests a weak surface, such as a casting skin or a lamination, rather than a plating fault.
- Identify the failed layer. On multilayer coatings, note whether the whole stack came off or only the top layer. A cross-section or a simple file test at the edge of the blister shows this quickly.
- Map the location. Edges and corners point to current density and burrs; recesses, threads and blind holes point to trapped solution; thick casting sections and gates point to porosity.
- Check the history. Did the blisters appear before or after a bake, a paint cure, soldering or forming? Did the machined parts sit in coolant or rust preventive for a long time before finishing? Ask the plater for the process route, the bake record and the adhesion test record for the lot.
- Test the parts that look good. Run the adhesion test the PO calls for on a sample of unaffected-looking parts from the same lot. If they blister or peel under the test, the weak bond runs through the lot and sorting by eye will not contain it; if they pass, the problem may be confined to a rack, barrel load or casting batch.
- Ask the plater to isolate the step. Two simple checks belong in the corrective action. A water-break test after cleaning shows whether a water-repellent film such as oil is still on the surface (ASTM F22-21). And a substitution trial, in which marked parts are hand-scrubbed with pumice after one cleaning step and run through the rest of the line beside unscrubbed parts, shows which step is failing: if the scrubbed parts adhere and the others do not, that step is the suspect (Altmayer, 2024).
Which ASTM B571 adhesion test should the purchase order require?
Choose the B571 test that imitates what the part will go through: a bend or draw test for parts that will be formed or crimped, a heat-quench test for parts that will be soldered, baked or run hot, and a file, scribe or chisel-knife test for rigid parts that cannot be bent. Then write on the PO which test, how many parts, when in the process it is run, and what result is a pass.
ASTM B571 is a practice, not a single test. It describes a family of simple qualitative methods and says the choice should follow the end use or fabrication method, with forming pointing to a bend or draw test and soldering or heat exposure pointing to a heat-quench test (ASTM B571-23). Besides bend, draw and heat-quench tests it describes mechanical tests such as file, chisel-knife, scribe-grid and tape tests, and its Table 1 marks which tests are appropriate for which coatings, so check that table before naming a test for a thin or brittle coating. Three clauses of B571 belong in every adhesion requirement:
- Agree the acceptance criterion. B571 says the test methods must be specified for acceptance inspection and the parties must agree on what is acceptable, especially for marginal results. “Adhesion per ASTM B571” alone leaves the pass condition undefined.
- Any failure fails. When more than one test is specified, failing any one of them is unsatisfactory.
- Test after the last thermal step. If the part is baked or heat-treated after plating, adhesion should also be checked after that step, which is where hydrogen and porosity blisters show up.
Many B571 methods damage or mark the part, so specify sampling rather than inspection of every piece, and say whether test panels are acceptable. B571 allows equivalent panels where the part’s size or shape prevents testing, provided they match the part’s material and surface finish and go through the same plating cycle. Many coating specifications, including ASTM B633 for zinc and ASTM B456 for copper plus nickel plus chromium, include adhesion among their requirements (ASTM B633-23; ASTM B456-17(2022)), so check what the cited specification already says before adding to it.
| What happens to the part after plating | B571 test to consider | What to write as the pass condition |
|---|---|---|
| Bent, crimped, staked or formed | Bend test or draw test | No peeling, flaking or lifting of the coating at the bend; cracking alone judged as agreed |
| Soldered, brazed, baked, painted and cured, or used hot | Heat-quench test | No blistering, peeling or flaking after heating and quenching |
| Rigid machined or cast part | File, grind-saw or chisel-knife test | Coating does not lift from the base metal at the filed or cut edge |
| Thin or decorative coating on flat areas | Scribe-grid or tape test | No coating removed between or along scribe lines |
| Part too small or complex to test | Equivalent test panels | Panels of the same material and finish, plated in the same cycle |
Should a blistered or peeling lot be stripped and replated, used as is, or rejected?
A lot that fails the specified adhesion test is nonconforming and should not be used as is for any part that is formed, heated, sealed or load-bearing. Stripping and replating is usually possible, but only after the cause is known, and it carries its own risks for high-strength steel, castings and tight-tolerance features.
Before approving rework, check three things. First, the base metal: acid stripping and re-cleaning of steel parts can introduce hydrogen again, so high-strength parts need the embrittlement relief the specification requires after replating, and the bake has to come before any conversion coating, as ASTM B850 notes. Second, the casting: if blisters came from porosity, replating the same castings tends to reproduce the blisters, so the fix lies in casting quality or sealing, not in the plating line. Third, dimensions: stripping can etch the substrate and replating adds thickness again, which matters on threads, bores and fits; the guide on threads after plating covers how coating build affects thread gauging.
Whatever the disposition, ask for the corrective action in writing: which cause was confirmed, which step changed, and how the next lot will be tested. Run the PO’s adhesion test on the first replated or new lot before releasing it to assembly.
What should the drawing and PO say so plating adhesion failures do not recur?
Name the coating specification with its full classification, the base metal and condition, the adhesion test method from ASTM B571 with sampling and pass criterion, the embrittlement-relief requirement where it applies, and the records you expect with each lot. Add the part-side controls that prevent the most common causes.
- Coating specification. The standard and its classification code or service condition, for example ASTM B633 with its service condition and type, or ASTM B456 with its service condition number, not a bare “zinc plate” or “bright nickel chrome”. The ASTM B633 guide shows how to read a full zinc callout.
- Base metal and condition. The alloy and temper or hardness, so the plater selects the right preparation route for aluminum, stainless steel, zinc die castings or hardened steel.
- Adhesion test. “Adhesion per ASTM B571, bend test (or heat-quench test, or file test), on a sample from each lot, after any post-plate bake. No peeling, flaking, blistering or lifting.”
- Hydrogen embrittlement relief. Where the steel is in the range the specification covers, require the bake and its timing; the guide on hydrogen embrittlement baking explains when it applies.
- Significant surfaces and contact marks. Mark where the coating must be continuous and where rack contact is allowed.
- Part-side controls. Break sharp edges and deburr holes; specify a cleanable rust preventive or none between machining and plating; for castings, state the surface quality required on plated faces and limit polishing that cuts through the casting skin.
- Records. Thickness results, adhesion test result, bake record where required, and the identity of the plating route, delivered with each lot.
How are plating defects handled on a Saola order?
On a Saola order, parts are machined at our lead CNC plant and plated at a partner finishing plant we manage, and the quote states the production site and country of origin. Plating defects are judged against the coating specification and adhesion test written on your drawing and purchase order, which is why those lines matter so much.
CNC turning and milling run at our lead CNC plant, T.K Viet Nam Precision Company Limited, in operation since 2017, as described on the CNC turning page. Electroplating, zinc plating, gold plating and trivalent chromate are applied by partner plants we manage, as listed on the surface finishing page; partner plants are not named and are not necessarily in Vietnam. A finish or test outside that menu is flagged at the quote stage rather than improvised. The split maps onto the cause list above: edge breaks, deburring, coolant and rust-preventive residue, and handling before parts leave for finishing sit on the machining side; cleaning, activation, strikes, bake and the adhesion test sit on the finishing side.
In practice, put the coating specification, the B571 test and the pass condition on the drawing or PO, and say which records you want with each lot. If a lot blisters or peels, send photos, the location on the part, the lot identity and, if you can, an opened blister or a lifted flake, and we take it up with the plant that did the work, using the checks in this guide to separate a machining cause from a finishing cause. ISO 9001 certification is in progress, not held. Send the drawing and finish notes through the quote form.
Frequently asked
Is a tape test enough to prove plating adhesion?
Rarely on its own. B571 says the test should follow the end use or fabrication method, so for parts that are formed or crimped a bend or draw test, and for parts that are soldered, baked or run hot a heat-quench test, is the closer imitation of what the part will see. Check B571 Table 1 for which tests suit your coating.
Why did the parts pass inspection and blister later?
Delayed blisters usually come from something trapped under the coating: gas or solution in casting porosity or blind features, or hydrogen in steel. They often appear after a bake, a paint cure or time in storage, which is why B571 says adhesion should also be tested after any post-plating bake or heat treatment.
Can blistered parts be fixed by replating over the defect?
No. Plating over a poorly bonded coating builds on the weak layer. The defective coating has to be stripped, the cause corrected and the part fully re-prepared before it is plated again.
Who is responsible when plated machined parts peel: the machine shop or the plater?
It depends on the cause, which is why the diagnosis matters. Burrs, sharp edges, heavy rust preventive, scale or porous castings start on the part side; incomplete cleaning, missing strikes, bad transfers between tanks and skipped bakes start on the finishing side. An opened blister and the process records usually settle it.
Does chrome peeling mean the chrome bath is bad?
Not necessarily. When chromium peels and the nickel below stays bonded, the usual cause is a nickel surface that became passive or contaminated before chromium was applied. When the whole stack peels from the base metal, look at cleaning, activation and strikes first.
Sources
- ASTM B571-23, Standard Practice for Qualitative Adhesion Testing of Metallic Coatings (scope and significance), ASTM International, 2023
- ASTM B322-99(2025), Standard Guide for Cleaning Metals Prior to Electroplating (scope), ASTM International, 2025
- ASTM B183-18(2022), Standard Practice for Preparation of Low-Carbon Steel for Electroplating (abstract), ASTM International, 2022
- ASTM B253-11(2022), Standard Guide for Preparation of Aluminum Alloys for Electroplating (scope), ASTM International, 2022
- ASTM B254-26, Standard Practice for Preparation of and Electroplating on Stainless Steel (scope), ASTM International, 2026
- ASTM F22-21, Standard Test Method for Hydrophobic Surface Films by the Water-Break Test (scope), ASTM International, 2021
- ASTM B850-98(2022), Standard Guide for Post-Coating Treatments of Steel for Reducing the Risk of Hydrogen Embrittlement (scope), ASTM International, 2022
- ASTM B633-23, Standard Specification for Electrodeposited Coatings of Zinc on Iron and Steel (abstract), ASTM International, 2023
- ASTM B456-17(2022), Standard Specification for Electrodeposited Coatings of Copper Plus Nickel Plus Chromium and Nickel Plus Chromium (abstract), ASTM International, 2022
- Polishing and plating zinc die castings, International Zinc Association, accessed October 2026
- Frank Altmayer, Troubleshooting Your Plating Process, Finishing and Coating, October 2024