Most plating failures are not plating failures. They are preparation failures that only become visible after the coating is applied, when the part flakes, blisters or rusts at a patch that was never properly receptive in the first place.
What the process of plating metals involves
Plating runs through a sequence rather than a single step. The part is inspected for surface condition, cleaned to remove oils, polishing compound and machining residue, rinsed, and then activated — a step that alters the surface chemically so the deposit can bond to it. Only then does the part enter the plating bath, followed by rinsing, drying and any post-treatment such as a bake or a passivation. Each stage affects the next, and skipping or shortening one shows up as a defect that appears to belong to the plating step.
| Stage | Purpose | Failure if it is inadequate |
|---|---|---|
| Incoming inspection | Identify scale, rust, porosity or damage | Defects are coated over and reappear later |
| Cleaning | Remove oils, residues, polishing compound | Patchy coverage, poor adhesion |
| Activation | Make the surface chemically receptive | Coating peels or blisters |
| Plating | Deposit the coating | Uneven thickness or burnt deposit |
| Post-treatment | Bake, passivate or seal | Embrittlement risk, corrosion at defects |
Why some metals need special pre-treatment
Aluminium forms an oxide within moments of being exposed to air, and that oxide blocks adhesion, so it is normally replaced with a receptive layer before plating. Titanium and some stainless grades need specific activation steps because their natural surface layers are chemically stable. Castings bring a different problem: porosity and entrapped contamination can release material during plating and cause blisters or pits. Plastics can be plated, but only after a conductive surface is created first.
The commercial consequence is that substrate identity determines how much preparation the job needs, and therefore how much it costs and how reliable the result will be. Two parts with identical geometry but different alloys are different plating jobs. Saying which alloy is involved, and how it was produced, is the single most useful piece of information in a plating enquiry.
What to send with a plating enquiry
Five items make a plating job quotable and repeatable. The substrate and its condition, including whether the part is wrought, cast or machined, and whether it has been heat treated. The coating and thickness or class required. The surfaces that must remain uncoated, because masking is a deliberate operation. The function of the coating — wear, corrosion, conductivity or appearance — since it determines which tests matter. And the dimensional condition for tolerances, before or after plating.
Two further details prevent rework. Mentioning any prior surface treatment, such as polishing, blasting or a previous coating, because residues and existing layers change the preparation route; and stating whether the parts have been handled with bare hands, since skin oils are a common cause of patchy adhesion on parts that were otherwise clean. Process guidance for these stages is published by the NIST Manufacturing Extension Partnership.
What is a cheaper alternative to chrome plating?
It depends on which property the chrome was providing.
If the requirement is hardness and wear resistance, an electroless nickel deposit with a suitable heat treatment is often the lower-cost route, and it has the additional advantage of coating complex geometry evenly. If the requirement is corrosion protection on steel, zinc-based coatings with a passivation layer are typically cheaper. If the requirement is decorative brightness, a thin nickel layer may serve, and if it is anodized appearance on aluminium, a conversion finish replaces plating altogether.
The substitution is cheaper only when it satisfies the same function, which is why the failure mode matters more than the finish name. A coating chosen for appearance will not survive a wear application, and one chosen for hardness will not provide the sacrificial protection that zinc gives to steel. Coating classifications and the tests used to demonstrate performance are published by ASTM Committee B08.
Common surface defects that originate before plating
Four defects account for most adhesion failures. Polishing compound left in a recess, because it is difficult to remove completely and it resists the cleaning step. Heat scale or oxide from welding or heat treatment, which has to be removed mechanically or chemically before activation can work. Embedded blast media from a previous finishing step, which creates a spot the coating cannot bond to. And handling contamination, where oils transfer from hands, gloves or packaging onto a clean surface.
The practical response is sequencing. Where a part will be polished, blast cleaned or heat treated, those operations should be completed and followed by a proper cleaning step before plating, rather than being treated as independent jobs. Specifying the sequence on the order prevents a batch arriving at the plating tank with a surface that was clean two processes ago, and it also fixes responsibility if adhesion later fails. Material and substrate references are published by ASM International, drawing conventions follow ASME standards, bath and waste obligations are set out by the US EPA, and the finishing options are described under surface finishing.
Send the part with the alloy, its condition and any prior surface treatment, and request a plating quote with the preparation route stated.
FAQ
What is the process of plating metals?
Inspection, cleaning, rinsing, activation of the surface, deposition in the plating bath, then rinsing, drying and any post-treatment such as a bake. Each stage depends on the previous one, and preparation decides whether the coating adheres.
What is a cheaper alternative to chrome plating?
It depends on the function. Electroless nickel often replaces hard chrome for wear resistance and coats complex shapes evenly, zinc-based coatings are cheaper for corrosion protection on steel, and anodizing replaces plating where the requirement is on aluminium.
What causes a coating to peel or blister after plating?
Almost always preparation: polishing compound left in a recess, heat scale not removed, embedded blast media, or handling contamination. Those create spots the coating cannot bond to, and they are prevented by sequencing cleaning after the other surface operations.