Microscale distribution of Ti-based conversion layer on hot dip galvanized steel

Cr-containing conversion layers for hot dip galvanized steel are currently being replaced with more environmentally friendly options. Generally, the hexavalent chromium pretreatments are known to provide excellent corrosion protection in most environments, while coil coated products pretreated with Cr-free alternatives are more prone to corrosion. Surface sensitive analysis techniques were employed on pretreated HDG in order to clarify the importance of mechanical distribution of a Ti-based pretreatment layer to corrosion protection. A correlation between the conversion layer evenness and the humidity resistance of the corresponding painted samples was observed. The studied Ti-based pretreatment layer was thus confirmed to provide protection mainly via barrier mechanism. Defects in the microscale distribution of the pretreatment layer are not detected by off-line X-ray fluorescence measurements that are commonly used for determination of coating weights of pretreatments on HDG. Automated EPMA measurements can provide valuable information on the distribution of trace elements in Cr-free conversion layers, which can be used as an indication of the conversion layer evenness and further utilized in process optimization. A transition from Cr-containing pretreatments to Cr-free pretreatments requires careful cleaning of the substrate and well managed application of the pretreatment film. However, when proper distribution and reaction of the pretreatment solution are assured, humidity resistance equivalent to that of Cr-containing pretreatments can be expected.