Zinc Nickel Coating Thickness for Salt Spray Resistance
The build thickness of a zinc nickel coating has a direct relationship with the endurance of a part in a salt spray test. Thickness therefore needs to be fixed in the coating specification, not left for the plater to decide. Although thickness is important, the passivation coating and any topcoat (sealer) matter just as much. A drawing with only a specified coating thickness is often the cause of a batch of parts failing its test.
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Typical thickness bands in practice
As a rough guide within the finishing industry, a zinc nickel layer of around 5 to 8 microns is typically sufficient for lighter parts, with typical neutral salt spray targets of 240 hours before red rust. For automotive and industrial fasteners, a typical zinc nickel layer thickness is 8 to 15 microns. Common targets for these parts are 500 hours or greater. Some aerospace and defence specifications demand 1,000 hours or greater. However, these figures are typical rather than guaranteed and can be affected by a number of factors including part shape, base steel and subsequent treatments.
| Typical deposit | Common use | Typical red rust target |
|---|---|---|
| 5 to 8 microns | Lighter duty, internal or sheltered parts | Around 240 hours |
| 8 to 15 microns | Automotive and industrial fasteners, brackets | 500 hours or more |
| To specification | Aerospace and defence | Often 1,000 hours or more |
How standards define acceptable performance
For corrosion tests specified in European corrosion specifications the neutral salt spray test (ISO 9227) is mostly used. In America corrosion tests are usually specified by reference to the very similar ASTM B117 salt spray (fog) test. The test is run in a chamber of warm salt fog and the panel or test article is inspected at pre-determined times and the number of hours before signs of corrosion appear noted.
There are two main types of corrosion. The first is the corrosion of the zinc alloy coating itself, i.e. white rust. This is expected to appear first. The second is the corrosion of the steel itself, i.e. red rust. This only occurs when the coating has been penetrated. Therefore, it is sensible to specify two hour limits for a part, the first for the appearance of white corrosion products and the second for the appearance of red corrosion products, as a part can show white corrosion products early whilst still offering excellent corrosion protection to the steel.
Zinc and zinc alloy coatings on steel with a chromium-free passivate are covered by the international standard ISO 19598, which sets out the naming of coating systems, the minimum film thickness and the minimum corrosion resistance the coating system must achieve in tests.
Where passivate and sealant treatments fit in
When planning to use zinc nickel coating it is commonly misunderstood that thickness is the only criterion to ensure acceptable performance in salt spray testing. A trivalent passivate applied over the deposit adds significantly to salt spray life, and a sealer or topcoat can take even a thin deposit well past its bare performance. Specifying zinc nickel without a passivate is unlikely to meet a demanding red rust target at the thicknesses normally associated with these specifications.
For Zinc Nickel Coating, see https://www.poeton.co.uk/surface-treatments/plating/zinc-nickel-plating/.
A few of the sealers have enough friction to affect the torque and clamp load on threaded fasteners. In such cases, the friction coefficient would need to be defined or an approved topcoat specified. Specify the required properties rather than letting someone else choose.
How to specify thickness on a drawing
A clear drawing note can save a lot of time and arguments when receiving plated parts. The minimum information that should be contained in this note is listed below.
- The coating: electroplated zinc nickel, with the standard it is to be supplied to, such as ISO 19598.
- The minimum local thickness in microns, and the significant surfaces where it must be measured.
- The passivate type, usually a trivalent or chromium-free passivate, and its colour if appearance matters.
- Any sealer or topcoat, including friction requirements for threaded parts.
- Salt spray hours to white corrosion and to red rust, and the test method, ISO 9227 or ASTM B117.
- Hydrogen embrittlement relief for high-strength steels, which the plater carries out by baking the parts after plating.
Confirming thickness and compliance
The thickness of the coating can be measured with X-ray fluorescence (XRF) or magnetic induction gauges, both of which are quick and do not damage the part. For items where the salt spray results are part of the acceptance criteria, agree whether the test will be witnessed, how many parts will be tested and what will constitute a failure before the coating process commences for the first batch. Failure is normally defined as the first appearance of red rust on a significant surface of the coating, judged against the criteria set at the drawing stage.
Common mistakes in zinc nickel specifications
- Stating a thickness with no passivate, then expecting several hundred hours to red rust.
- Giving a single salt spray figure without saying whether it means white or red corrosion.
- Forgetting that recesses and internal threads plate thinner than flat outer faces.
- Leaving out embrittlement relief on hardened fasteners.
- Specifying a sealer without checking what it does to thread friction.
For answers to the questions buyers ask most often, from colour options to how zinc nickel compares with plain zinc, see our zinc nickel plating FAQs.
Nailing a correct specification for test and verification is crucial in removing ambiguity with a plater. Settle thickness, passivate, sealer and test criteria on the drawing, and the salt spray result becomes a confirmation rather than a surprise.
