Outside the Box Thinking on Passivation

Q: We have inconsistent salt spray results from our zinc line. The concentration, pH, temperatures, and metals are routinely monitored in the passivate. What could we be overlooking?

There are several areas that can impact the performance of zinc and zinc alloy passivates and therefore the protection they provide which have nothing to do with the pH or the concentration in your process tank. Problems within the process itself are usually easily diagnosed. However, addressing problems from elements that are not directly related to the passivate takes a better understanding of the entire process. 

Zinc and Zinc alloy passivates require a continuous hydroxide film to provide an additional layer of protection to the plated deposit. The condition of the passivate film depends largely on the plated deposit underneath. The plated layer needs to be bright, blemish-free, and at least 8 microns thick for high-performance passivation. In the passivate solution, there are several chemical reactions taking place. If there are visible pits, burns or other imperfections within the plated deposit, there will be decreased reaction between the surface of the part and the chromate solution. This will create a thinner than desired passivate layer, or if the condition of the plated layer is bad enough, sometimes no passivation at all!

Moving down the process line, the post plate rinse tanks can be the cause of passivation issues in an acid zinc process specifically. Acid zinc plating solutions are high in corrosive salts and organics which accumulate in the rinses after the plater. These materials will form a film on the parts prior to passivation which slows or stops the chemical reactions at the interface, causing defects in the passivate film. This issue may manifest more frequently if a rack or barrel is left in a post plate rinse during downtime. The parts will accumulate organics and salts on the surface until they appear white with a film on them. These rinses need to be dumped, cleaned and made fresh at least weekly.

The last step of the plating process that could be impacting your performance is the part drying. There are two components to drying that are important to passivation performance. The first is the removal of any excess process water on the parts. Residual water in blind holes, welds or other areas can react with the passivate layer, especially if the water contains chloride ions from city water or contamination in a dirty final rinse. Secondly, trivalent passivation films are “hydrated” right out of the process tank, which then need to be “cured” to properly harden and give you a continuous coating protecting the plated surface. The temperature of the dryer is important to monitor in order to keep the temperature of the part below about 175ºF. Above this temperature, the passivation film will be exposed to the high temperature for too long. If the passivate film reaches such a temperature that it dehydrates too quickly, the film will rapidly contract and crack, exposing the substrate and decreasing the effectiveness of the film. 

A final failure mode for passivation comes from part handling. Material handlers taking parts on and off racks with dirty gloves or without gloves at all may damage the coating before it has the chance to cure completely. Even the employees packing up the samples for shipment to the testing lab or the packaging itself can cause issues with salt spray once it goes into testing. Parts being tested for corrosion resistance should be packaged with VCI paper or equivalent and should not be put in resealable plastic bags or wrapped in shrink wrap. Plastic barriers lock moisture in with the parts and will accelerate the formation of corrosion cells. 

These areas outside of the passivation step of the process may not be immediately evident to the person who is troubleshooting performance issues. Great corrosion resistance comes from having a clean, well maintained process from start to finish. Each step must be completed properly for the next step in the process to work as expected. Keep an open mind when looking at problems to determine all the factors that can contribute to the issue, even ones outside of the process in question. Once you start tracking these elements backwards, usually the solution becomes evident quickly and the process can be pulled back into control.