Corrosion Resistance Problems

Q1. I stumbled across your forum while Googling. We are manufacturing a line of insulated pipe guides and slides with a B-117 1,000 hr powder coat surface. Many mechanical specifications insist on hot-dip galvanizing for steam tunnels or outdoor applications. Is there any side by side comparison between the two methods? M. R.

Q2. We are a manufacturer located in NW Iowa. We supply heavy truck cab suspensions to OEM customers. One of our customers requires our finishes to pass a 365hr salt spray test on both flat and curved sections. We pass the test on flat parts but fail on the curved. We have been working with our suppliers on this and have tried several different things over the past few months but again failed the latest testing. We would be interested in talking with someone about possible strategies to try to achieve a passing result and improve our process. Thanks. J.W.

A. Following is an answer from my August 2006 column that seems to apply here, as well.

Corrosion resistance is primarily achieved by a combination of substrate selection, pretreatment chemicals, occasionally primers, and a good topcoat. You can obtain the same corrosion resistance by using these attributes in different combinations, or all of them at once, depending upon the particular corrosion requirement.

Since your product substrate is selected for reasons other than corrosion resistance, you must compensate with pretreatment chemicals and primers and topcoats. You may even achieve your corrosion requirements using just the proper pretreatment and a single topcoat. Following is our experience on your substrate (cast ductile iron) with the choices you have available:

No pretreatment and powder coating topcoat	100+ hours
No pretreatment; e-coat primer and powder topcoat	100+ hours
Iron phosphate and powder coating topcoat	500+ hours
Zinc phosphate and powder coating topcoat	750+ hours
Iron phosphate and e-coat primer and powder coat topcoat	1,500+ hours
Zinc phosphate and ecoat primer and powder coating	2,000+ hours

As I always recommend, you should test your particular selection(s) prior to final implementation.