Need corrosion comparison, galvanneal vs. unplated steel

A. Salt spray is a start, I suppose, but if you told us the specific application, you might find someone with just the experience you need. For instance, if you were interested in this material for roofing, I would send you to Englert, Inc. Perth Amboy, NJ , since they coil coat and install metal roofing.

A. Your other respondee is correct - how much corrosion occurs depends on the environment of the application. However, here are some general thoughts for you. Plain, unprotected steel will start to develop small rust spots across its surface and at edges in a few days just from moisture in the air. If the steel goes through periodic wetting, such as from rain, etc., the rusting is drastically accelerated. It is now known that salt fog-type tests do not say much about the rates of corrosion unless you are interested in a salt water environment. In fact, the salt fog tests actually produce a totally different type of rust and corrosion reactions than occur in most real life situations. Galvannealed steel has a hot dip zinc coating that is then annealed (heated) so that there is exchange of zinc atoms in the coating with iron atoms in the substrate. For this reason, galvanneal will relatively quickly get a brown cast to its surface, but this turns out to be a hard durable thin layer that does not penetrate into the rest of the coating. Edges are protected, even without zinc directly on them, by cathodic protection - the nearby zinc preferentially corrodes instead of the steel. If your application is left unexposed, then you may want to consider other types of galvanized steel such as electrogalvanized and regular hot dip galvanized. These coatings do not contain iron and will only produce so-called white rust in which a thin zinc oxide forms on the zinc itself. If you are painting the material, then galvanneal is fine if you paint it quickly before a lot of the brown surface appears to interfere with paint adherence. Galvanneal coatings are usually a bit more brittle than the other types of galvanized so if you are bending it severely or forming it you may have flaking off of the coating that would not occur with electrogalvanized on modern minimum spangle hot dip galvanized. Galvanneal, however, does weld a bit easier and gives longer electrode life if your welding equipment is set up for it.

A. Hi, Rick. G90 means 0.90 ounces of zinc per square foot (divided between the two sides). More or less zinc can be applied whether the galvanizing is later annealed or not. Typically, Cold Galvanizing Compound / Zinc Rich Paint is applied where the galvanizing has burned off.

Regards,

A. Hi, Kevin. As Dennis noted above, salt spray results are pretty meaningless for any predictive value, especially for galvanizing and galvanneal where the real-world corrosion resistance is dependent upon the longterm formation of zinc carbonate reaction products which don't form in salt spray testing. I don't have test results, but you will see some advice in letter 5633. Good luck.

Regards,