Harsh conditions prevail in many production plants. Components made of stainless steel have to withstand a lot on a daily basis. In the process industry, for example, the material surfaces of the pipes and containers, which are usually made of chrome-nickel-molybdenum steel, come into direct contact with acids, corrosive gases or other aggressive media.

The protective passive layer consisting of chromium oxide on the steel surfaces has a decisive influence on the economic and safe operation of the system. However, due to faulty or unfavorable conditions during production, processing, transport or storage, the surface protection on the components may be damaged or not continuous. If the passive layer is defective, there is a risk of localized corrosion, including pitting corrosion. This can result in material leaks, hazards for people and the environment, unplanned plant shutdowns and costly repairs.

Damage to the passive layer is not visible to the naked eye. Conventional methods for checking the protective layer, such as salt spray tests or electrochemical measurements, represent a considerable financial hurdle for small and medium-sized companies. This is because they require time, special equipment and expert knowledge. What was missing until now was an easy-to-use test method that did not require any special prior knowledge to use. The so-called KorroPad rapid test closes this gap.

TÜV Süd Chemie Service routinely uses electrochemical measurement methods to check the condition of passive layers. For example, current density-potential curves are recorded in laboratory measuring cells or locally on components. From this, pitting corrosion potentials can be determined and thus possible corrosion risks can be detected. It was therefore of particular interest to compare methods: The experts wanted to compare the results of electrochemical investigations and the tests using the KorroPad method. The aim was to evaluate the rapid test developed by the Federal Institute for Materials Research and Testing (BAM) and test it in practice.

Iron ions reveal weak points

The KorroPad method works in a surprisingly simple way: if the protective chromium oxide layer on the steel surface is missing, iron ions escape from the material. This is where the test procedure comes in. The gel-like KorroPads consist of water with small amounts of sodium chloride and an indicator for iron ions.

The indicator potassium hexacyanoferrate(III), which is yellowish-transparent in aqueous solution, changes color to "Berlin blue" on contact with the iron ions. Clearly visible blue dots appear as indicators on the pale yellow pads. At these points, the protective passive layer on the stainless steel surface is not present or could not form.