Metal finishing Q&As since 1989

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Corrosion of aluminum with EN coat in wet environment

We plate some food machinery parts that are made from a variety of Aluminum alloys for corrosion resistance. In food industry rinse with alkaline soak is common and this alkaline and wet environment damage the Al parts of packaging machines. For this reason we plate parts with EN bath with medium content of P (6-8%) thickness of 20 micrometer and after soak clean and etching we use two step alloy zincate (zinc-nickel) and after rinsing (two step)parts are plated with acid electroless nickel. After plating, parts are, for better adhesion baked at 200 °C . The adhesion of En layer tested and was good. but what's problem? After six month ** with out work of the machine ** corrosion in all over the EN plated parts could be seen (only EN plated "for example this kind of corrosion can't be seen on AL parts with out coat"!). The corrosion of Al is very very coarse and rough and the nickel coat blister in corroded area. I test some Al-En coat in strong alkaline and acid environment but I can't see this kind of corrosion after 40 hours.

Please tell me WHAT'S PROBLEM?

Vahid Hosseini
- Tehran, Iran
2003

The problem with aluminum in this application is that any pits, scratches or dents that expose bare aluminum the aluminum will be attacked by the alkaline/caustic cleaner. The aluminum will dissolve under the nickel causing it to flake. There are cleaning products that will not attack aluminum. Also you should use a high phosphorous electroless nickel with 11% or more phosphorous.

Todd Osmolski
- Charlotte, North Carolina, USA
2003

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Dear friends,

After searching about what cause this problem and see the other machine of this type I find out this kind of corrosion can't be seen so I search about cleaners and I find that the use of hypochlorite sodium as bleaching and anti bacterial agent can cause this type of corrosion. the use of NaOH in formulation in this type of bleaching agent may cause severe corrosion on Al. Bout after testing a foil of EN medium P(6-8%)in hypochlorite bleaching agent I see that the foil decompose after 2 hours bout a precipitation of black nickel formed on bottom the testing tube. I though that the Phosphorus in the matrix of plating layer oxidize with hypochlorite and decompose the metallic layer. If I right please tell me.

Vahid Hosseini
- Tehran, Iran
2003