Chemical Conversion Coatings

Question:

I am looking for a conversion coating that will withstand a 1,000-hr exposure in the 5% salt spray test. I am familiar with Mil-C-5541E, Chemical Conversion Coatings on Aluminum and Aluminum Alloys, but this specification describes a specification that will withstand less than 200 hr in the 5% salt spray environment. Can you give any information on coatings that can withstand a 1,000-hr salt spray exposure? S.K.

Answer:

Your requirement of 1,000 hr is a rather “tall order” to fill. I would start out by talking to some of the chemical vendors listed in the Suppliers section at www.pfonline.com. Go to the Suppliers page and click on “Cleaning and Pretreatment,” then Chromate Conversion Coatings.” Also, these companies can be found in the 2000 Products Finishing Directory and Technology Guide under Chromate Conversion Coatings. No doubt they can recommend some proprietary systems that will give more than 200 hr in the salt spray test.

There have been a number of attempts in the last few years to develop conversion coatings for aluminum that can withstand a longer period of time in the salt spray environment. One promising system consists of two separate steps: 1) An activation layer is formed over of the aluminum oxide surface of the part; and 2) A stable inorganic polymer is formed on the surface by applying a proprietary sealer. This system is described in a paper, “Coating Systems that Compete with Chromates,” presented at the 19th AESF/EPA Conference, February, 1998. The paper presents data showing that 3003 aluminum can withstand more than 1,000 hr exposure in the ASTM B117 salt spray test without pitting. I have e-mailed you the name of the company.