Failure Salt Test Hard Chromium Plating
Q. Why doesn't the cylinder with a 80 microni chromium layer (plated on hard chrome bath with catalyst heef 25) resist on salt spray test (occurs red rust)?
A. Hard chromium is generally used as an "engineering" coating, and by itself not well suited for "decorative" or corrosion free applications, except when applied directly to Stainless Steel. Hard chrome plating is better suited for restoring worn dimensions, producing high hardness and wear resistance.
To achieve a higher degree of corrosion protection using hard chromium 1.)Hard chrome may be deposited over a layer of nickel. 2.) follow a process known as "Duplex" hard chrome plating.
Hard chromium plating goes down on the base metal in the form of small platelets in a random formation, also termed micro-discontinuous. Forming tiny pathways between the layers. Further, as the chromium begins to build in thickness, micro cracks form. A third phenomenon is chrome hydride decomposition which leaves microscopic voids.
Any one or a combination of these facts about hard chromium can lead to the formation of corrosion.
Best of luck.