Development of Trivalent Chromium Passivation for Zn Platng with High Corrosion Resistance after Heating

Trivalent chromium passivation is used after zinc plating for enhancing corrosion resistance of parts. In the passivating process, the amount of dissolved metal ions (for example zinc and iron) in the passivation solution increases the longer the solution is used. This results in a reduced corrosion resistance at elevated temperatures. Adding a top coat after this process improves the corrosion resistance but has an increased cost. To combat this, we strove to clarify the mechanism of decreased corrosion resistance and to develop a trivalent chromium passivation with a higher corrosion resistance at elevated temperatures. At first, we found that in parts produced from an older solution, the passivation layer has cracks which are not seen in parts from a fresh/new solution. These cracks grow when heated at temperatures over 120 degrees Celsius. Next we researched the reason for cracks to occur and found that the main difference between an old and new solution’s layer is the amount metal deposits in it. These metal ions deposit into the passivation as hydroxides, and the larger the quantity in this layer the more the layer contracts by heating, meaning the newer the solution the less the layer contracts. So, we investigated developing a new solution to improve the corrosion resistance after heating through the reduction of metal ion deposits in the passivation layer. We achieved this reduction by adding organic carboxylic acid to chelate the dissolved metal ions. The carboxylic acid prevents excess depositing of these ions in the passivation layer. Using this developed solution, cracks disappeared and the corrosion resistance after heating was improved.