Corrosion Blisters

Q. After 500 hours of salt spray SS testing of our electrocoated parts, we see many blisters but little corrosion increase from the test X-Cut. Our parts are made of cold rolled steel CRS with 25µ of black epoxy e-coat paint. What causes the blisters if the overall corrosion protection of the part is good? What can we change in the process to make it better? R.M.

A. Blisters are one of the major corrosion problems encountered by automotive companies in the field after parts have been electrocoated. Many corrosion analysis and root cause investigations of failed salt spray SS parts by blisters indicate that most common root causes are poor part cleaning and/or rinsing. Both mechanisms leave oils or salt deposits on the part surface that get trapped before the final electrocoat is applied. Those residues, when exposed to the various environmental chemical conditions in the field, or salt spray solution SS in your situation, react with the chemicals in the testing solutions, causing swelling of the electrocoat at the surface. Potential solutions to reduce or eliminate blisters in electrocoated parts could involve one or several of the following:

1) Improving cleaning capability. Cleaning is always one of the most critical steps in the electrocoat process. Ensure that the cleaner you use is the most compatible with the type of oils and contamination present on your parts, and that you maintain the proper concentration, temperature, level of impingement and agitation. Also, you have to make sure you maintain a good dumping cycle for the specific rates and loads of your process. Your pretreatment supplier should be able to help you optimize your cleaner so that the parts are properly cleaned. Ensuring your parts are properly clean is a guarantee your finished product will maintain the corrosion protection needed for your specific application.

2) Improving rinsing and rinse conductivity. The rinses help in removing the residues of process chemicals used prior to electrocoat (cleaners and phosphate).

To minimize the amount of chemical residues left during rinsing, it is recommended that you establish the proper overflow rate and maintain as low of a rinse water conductivity as possible. Typically this is accomplished by using online conductivity meters, automatically feeding fresh or DI water, depending on the rinse tank, and by implementing counterflow rinsing.

By using automatic conductivity meters with a control set point (eliminating the conductivity highs and the lows), the rinse stages are maintained with more consistency. Typically the rinses prior to phosphate are made up of towns or city water, whereas the rinses after the phosphate stage are made up of DI or RO water. Care must be taken to ensure effective rinsing without excessive water use. While it is not always economically feasible to heat or cool a rinse stage, the effects of temperature on the effectiveness of rinse stages should be taken into consideration.

3) Improve or start using your process sealer. Sealers are used as one of the final rinse stages to aid in the prevention of blisters. The sealer passivates any bare metal left by voids in the phosphate coating, as well as neutralizing or inactivating any salts left on the parts by process chemicals from previous stages. The end result is that a sealer further improves a substrate’s corrosion resistance. To obtain the most out of your sealer, it is important to maintain the concentration and pH levels of the sealer bath.

4) Improve final rinse conductivity. Ensure that your last water rinse—after sealer and before electrocoat—is made up with DI or RO water and that the conductivity does not exceed 50 microhms/cm. This will ensure your parts entering the electrocoat tank are clean and without any residues that will be detrimental during corrosion testing or outside exposure.

Regards and good luck!