New rapid test for stainless steel surfaces

KorroPad is the name given to a new rapid colour test developed at BAM Federal Institute for Materials Research and Testing that enables stainless steel surfaces to be tested inexpensively and rapidly and, most importantly, also by non-specialists. The pleasure of many designs or components made of stainless steel sometimes declines after a short time. Instead of a beautiful metallic shininess, discoloration and stains may develop after a few months. A few optimists like the emerging 'leopard pattern' or lively surface, but most customers simply complain because the appearance does not meet their expectations.

The unexpected discoloration and stains can easily be explained: the protective chromium oxide layer - also called a passive layer - fails to fully develop. However, the stainless steel owes its name solely to this layer. This layer cannot be seen, thus, its defects cannot be recognised. There are a number of reasons for the protective layer to become impaired. The problems are often caused as early as during processing. But faulty semi-finished products, which are currently manufactured world-wide, lead to nonconformities in the materials' expected corrosion resistance.

The KorroPad test enables steel surfaces to be checked immediately after processing or in the delivery state. In this way, for example, handicraft businesses can protect themselves against costly warranty claims. And this not only involves visual problems since stainless steels are also the material of choice for the manufacture of anchors, dowels and bolts, hazardous material containers and complex chemical plants. These are usually installed in such a way that an error is not obvious at the time, which can lead to safety problems. Thus KorroPad also serves safety in engineering.

Three pads are needed for a test which provides a snapshot of the state of the passive layer. The pads are about the size of a five cent coin and are placed on the stainless steel surface. First, the surface is cleaned with acetone or alcohol and the pads are then lightly pressed onto the surface to be tested. They are removed with a spatula after 15 minutes and placed on a plastic support film where they can be scanned or photographed.

15 minutes is very fast compared to other methods which often take several hours or even weeks. KorroPad also offers another advantage: the component can be further processed after the test or installed at the customer's premises because it is virtually non-destructive.

Water and a ferric ion indicator are the ingredients of the gel-like pads. If the protective chromium oxide layer is missing on the steel surface, the indicator reacts with the iron ions in the material. The outcome is that the indicator changes its colour and blue dots appear on the slightly yellow pads. Each point indicates a spot on the steel surface where the protective passive layer was not able to develop.

Stainless steels consist of at least twelve percent chromium. The protective layer on the surface needs oxygen and water (from humidity) and a clean, bare metallic surface to be able to develop. The passive layer then seals the surface. If the protective layer cannot be formed all over the surface, corrosion occurs.

"KorroPad helps people help themselves," says Andreas Burkert who, together with Jens Lehmann, has developed the test at BAM. Visually the surfaces always look blank at the beginning. But do the components really provide what the material's name promises in practice? Many factors play a crucial role: How has the surface been processed? How have the weld seams been treated? Are the alloying elements evenly distributed? Burkert continues: "It is like baking a cake: The ingredients alone are not enough! The dough needs to be stirred properly and the subsequent heat treatment in an oven must be at the right temperature over the right time."

Meanwhile, some companies have already used KorroPad. The quick method is also included in the teaching curriculum in vocational schools and at universities. KorroPad visualises material properties that are otherwise hidden to the human eye. And KorroPad shows students how important it is to adhere to the rules of processing. Because standards alone will not lead to a safe technical application, what counts is what actually has been implemented in practice.

Source: Bundesanstalt für Materialforschung und-prüfung (BAM)