KorroPad An easy to use tool for testing passivity

An easy-to-use test kit is now available, which makes it possible for non-experts to find out, quickly and cost-efficiently, whether or not a stainless steel surface is in the passive state.

A research team of BAM, the German Federal Institute for Materials Research and Testing, has developed translucent gel pads the size of a coin. They contain a chemical indicator that turns blue upon reaction with iron. Stainless steel in its normal passive state does not have any free iron at the surface: a complex, invisible chromium-rich oxide layer known as passive film prevents the alloy from reacting with its environment. If iron is found on the surface, it indicates imperfections of the passive layer or contamination.

The starting point of the research project was the observation by metal builders that some polished decorative tubes and bars developed “unexplained” tea staining. Expert testing confirmed that a suitable grade had been selected for the given environment, the chemical composition of the stainless steel was in line with the standards, welds had been cleaned properly and external sources of iron contamination could be excluded. So what had gone wrong?

In-depth research revealed that the surface conditions require attention. For instance, when stainless steel surfaces are polished with coarse abrasives, excessive pressure or high speed, the surface can be damaged. The resulting defects may be imperceptible to the unaided eye but can make the surface susceptible to corrosion at a later stage. Microscopic recess areas are particularly vulnerable. They can develop crevice corrosion, a process that locally prevents passivation, even in environments that would normally be uncritical for the selected alloy.

The KorroPad method also identifies other types of defects, which can make a stainless steel surface susceptible to corrosion, for instance poor welds, inadequate post-welding surface finishing or iron contamination. On areas, which are not passive at the time of the test, the pads will turn blue.

The test is easy to use. First, the surface to be tested is cleaned with alcohol or acetone. Then the pad is applied and left on for 15 min. It is subsequently removed with a spatula and placed on a transparent film, which makes it easy to scan or take photographs of the results for documentation. The test is non-destructive and does not leave visible marks on the product. The pads work within a wide temperature range and fresh pads will have a shelf life of at least six months when stored refrigerated.

KorroPad is the result of a co-operative research project, of which the German stainless steel development association, Informationsstelle Edelstahl Rostfrei (www.edelstahl-rostfrei.de) was a partner. Test kits including 100 pads and refills of 30 pads are now available from the BAM web shop.

Improper polishing is a common cause of microscopic debris and recesses, which can make stainless steel surfaces prone to tea staining.

The coin-sized KorroPad indicates non-passive areas, which are susceptible to localised corrosion, in blue. Photos: BAM, Berlin (D)

Published 20/06/2014 14:22:16 Last Modified 20/06/2014 14:25:07

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