Corten A Unveiled: Your Ultimate Corrosion Barrier

 

Do you know the Corten A steel plate has the corrosion resistance?

‘Corten’ or ‘weathering steel’ comes in two flavours: Corten A and Corten B. Both are claimed to exhibit corrosion resistive performance but Corten B is alloyed for greater strength (but in so doing perhaps compromises its corrosion protection behavior to some degree).

The way the material works is that small amounts of copper and other elements change the nature of the rust that forms on its surface when subjected to atmospheric corrosion by making the corrosion product very finely grain-sized and very tightly adherent to the steel surface. The corrosion product then behaves like a self-generated paint coating that smothers subsequent attack and therefore slows the corrosion rate.

However, it has to be used cafefully: during the early months of use it Must be subjected to alternating wet and dry exposure in order to form the protective corrosion product. Failure to enable this to happen risks disruption of the rust layer, which then may no longer be protective. The material may then corrode even faster that plain carbon steel because of the minor element constituents. So, for example, if it is used unpainted for a road bridge, but the prevailing atmospheric conditions mean that the underside does not dry out frequently or completely, then corrosion can proceed unabated because a coarse non-protective rust deposit is formed instead of the desired fine adherent self-generated coating.

There is no doubt that, when used in suitable conditions, Corten can perform spectacularly well and painting can be avoided for many years, thereby offsetting the higher initial cost of the material. However, it is equally certain that Corten was over-sold from time to time in the past, especially outside the USA, in locations where the prevailing atmospheric conditions differ to the inland dry service environment that typifies the Southern and midland states. In cooler damper locations the asset owners find that Corten fails to produce its protective patina, in which case, there is no alternative to grit-blasting and painting as for any other steel bridge. Locations that use it also have to accept that there will be a brown rusty run-off that can be unsightly if specific provions have not been taken to get rid of it before it stains nearby walkways, roadways, parking areas, etc.

One other application where Corten can perform extraordinarily well is in Ljungstrom-type air heaters for power generation systems, which can be susceptible to high rates of attack due to sootblower erosion. However, because the affected area goes through a wetting/drying cycle every time the sootblower are used - perhaps every four hours, or at least once per shift - the protective patina has an excellent opportunity to form, and the service life to the air heater plates is extended considerably.