Q. We make steel cabinets that are used outdoors and meet a salt spray standard of 500 hrs. We are able to pass the salt spray test, but we have had some field failures in which rust occurs, usually on edges, but sometimes also in corners and on flat areas of the part. We use a five-stage iron phosphate wash process and a TGIC powder coating material. Any suggestions on how we can ensure corrosion resistance?—B.A.
A. As you have learned from your experience, salt spray results and field performance are not closely connected. Salt spray will help compare different pretreatment methods and coatings, but it does not tell much about the corrosion resistance of a part over time. It can also provide input on the stability of the process. If we pass a certain level of salt spray one day and do not pass on another day, we may have something that is not working properly.
Remember to always use a control panel from the test facility to be sure the problem is not related to your steel. A polished ACT or Q panel should always be part of your testing. You also need to think about the coating process that is needed to get good performance on steel in an outdoor environment. Iron phosphate does not add much to the corrosion resistance. I would suggest you consider a zirconium oxide or zinc phosphate to improve performance. The zirconium oxide will improve corrosion resistance and does not have the environmental challenges of the zinc phosphate.
Before you begin the chemical treatment, be sure you have clean weld areas, and round the sharp edges if you can. The chemical pretreatment will not remove weld smut and it sounds like you need better edge coverage. You can grind or blast those inorganic soils and you will get better performance.
You should also consider a powder primer coat. The primer will add a lot of corrosion protection and will also help to round those edges and give you more coating at these critical locations. A double coat of powder is always superior for edge coverage and corrosion protection.
Finally, if you want to know how your part is going to hold up, you should consider a cyclic corrosion test. Putting a part through wet/dry cycles will provide better correlation to the real world. Different specifications exist or can be created as to the specific cycles to use for a given industrial application. SAE J156315 is a good source for guidance on cyclic testing.