The thickness of a zinc nickel deposit has a significant influence on the duration of test samples in a salt spray test. It is therefore very important to specify the thickness of the deposit from the beginning.
Typical Thickness Bands in Practice
The typical thickness for zinc nickel work is between 5-8µm and is suitable for lighter applications with a target of 240 hours in a salt spray test. An 8-15µm thickness is more typical for automotive and industrial fasteners and has a target of 500 hours or more in a salt spray test. Some aerospace specifications demand 1,000 hours or more.
How Standards Define Acceptable Performance
Most European specifications for corrosion protection are based on the neutral salt spray test described in ISO 9227. These test methods are also widely used within the American industry and are described in ASTM B117. In both cases the results of the test are reported by measurement of the red rust appearance on the test specimens.
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 zinc nickel deposit can add 200-400 hours of extra life in salt spray testing, and a sealant topcoat can go on top of the zinc nickel coating to achieve more than 1,000 hours of resistance to red rust in salt spray testing, even on the thinnest of deposits. Therefore, specifying zinc nickel coating without a passivate treatment is unlikely to meet the stringent red rust criteria of demanding salt spray targets at the typical thicknesses normally associated with such specifications. For Zinc Nickel Coating, see https://www.poeton.co.uk/surface-treatments/plating/zinc-nickel-plating/.
How to Specify Thickness on a Drawing
Confirming Thickness and Compliance
The thickness of the deposit is confirmed by either X-ray fluorescence or magnetic induction measurement. For salt spray testing, the test is to be conducted as a witnessed test to the agreed hours. Failure is defined by the appearance of red rust in accordance with the criteria agreed at the drawing stage.
Nailing a correct specification for test and verification is crucial in removing ambiguity with a plater.