How to Improve Liquid Corrosion Resistance
Q. How can we improve the desired corrosion resistance expected from our coating?
A. Pretreatment is essential for optimal corrosion resistance, but assuming you have this under control, I would look at three additional areas:
- Coating application. Corrosion starts at the weakest link on the part, typically on an edge or in a corner where the thinnest film of coating can be found, which in turn becomes the least amount of protection. It will be important that you work with your applicators technique to get coverage in those most vulnerable areas to assure optimal protection.
- Consider a primer. If you believe you are doing the best you can with applicator technique, then you may need to consider a primer or discuss with your coating supplier a formulation that will allow for better edge coverage. Either option can help, keeping in mind that the goal is to protect those areas where the coating has a difficult time building film. In the case of a primer, this will not only give you two layers of protection but will enhance the topcoat’s ability to build, as the primer will reduce the sharpness of the edges on the part.
- Product design. One additional consideration, and often overlooked, is product design. Was the product designed for a particular coating? Does it have difficult if not impossible to reach recessed areas or welded areas hiding raw surfaces where a coating cannot reach? Was the product or part designed in a way that will trap moisture or hold water when exposed to an outdoor atmospheric condition? If these areas can be modified to better accept a coating then changes should be considered.
You must start with a clear understanding of where the corrosion is starting and then work backwards. Also make sure you have an understanding of corrosion expectations. If you are seeing corrosion after five years in a harsh outdoor environment, perhaps your expectations are too high for your coating choice. Make sure that whatever you do, you have agreed corrosion specification that can be measured and quantified so that nothing is left to subjectivity. Cyclical corrosion testing or ASTM B117 salt spray tests are a couple of options to consider when selecting a tech method for measuring corrosion resistance.
Originally published in the January 2017 issue.
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Better adhesion, enhanced corrosion and blister resistance, and reduced coating-part interactions make pretreatment a must.