CHEMICALS & PHARMACEUTICALS INDUSTRY FOCUS QUICK TEST DETECTS DEFECTS IN THE PASSIVE LAYER
The passive layer of stainless steel system components used in the chemical industry is often at risk from the harsh industrial routines. Dr. Helga Leonhard, from TÜV SÜD Chemie Service, and Jens Lehmann, from BAM (German Federal Institute for Materials Research and Testing), discuss how KorroPads can provide valid information on the passive layer's corrosion resistance within 15 minutes
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n chemical production, pipes and containers made of stainless steel are exposed to rough environmental conditions. This includes contact with acids, corrosive gasses or media containing high levels of chloride. The condition of the protective passive layer on the stainless-steel surfaces is directly connected to technical safety and the profitable operation of a production facility. When there are imperfections in the passive layer, localised corrosion or even pitting is possible. However, it's not visible with the naked eye where the passive layer is incomplete. Conventional methods of testing the protective layer, such as salt spray tests or electrochemical measurements, pose a large financial challenge on small and medium-sized enterprises. And special equipment and expert knowledge in the field of corrosion chemistry is also required. TÜV SÜD Chemie Service routinely uses
electrochemical methods for the safe selection of materials. For example, in laboratory measuring cells or at specific locations of components, the pitting potential is determined with current density-voltage curves. Based on this experience, it was of particular interest to compare the methods of electrochemical measurements and testing with KorroPads. The fast-track procedure was to be tested in practice in order to provide users with an easy-to-handle on-site method where suitable. In plant engineering in the chemical industry, stainless austenitic chrome- nickel-molybdenum steel is often used. Chrome is the most important alloying element for corrosion resistance since it creates a dense chromium oxide layer on the steel surface from an alloy content of approximately 10.5%. This passive layer is only a few atomic layers thin, invisible and sensitive. The risk of corrosion exists when the passive layer has not yet been properly formed. But also, when the material surface contains imperfections that prevent passivation. In the event of
TÜV SÜD steam pressure technology
(Image credit: TÜV SÜD)
damage, the passive layer can form again. However, this only happens when the physical and chemical prerequisites for a new formation are given. Decisive factors are sufficient oxygen concentration, moisture, low concentrations of chloride ions, as well as clean and smooth metallic surfaces.
BAM KorroPad schematic diagram: KorroPad rapid test: As an indicator, clearly visible blue dots appear on the light yellow pads. At these locations, the protective passive layer does not exist on the steel surface
(Image credit: BAM)
KorroPads provide an economic and fast alternative to testing methods applied so far. Water with low amounts of sodium chloride and an indicator for iron ions are the components of the gel-like KorroPads. When there is an incomplete protective chromium oxide layer the indicator potassium ferrocyanide(III), which is yellow-transparent in a watery solution, shows a spontaneous change in colour to "Berlin blue" when in contact with bivalent iron ions, which leak from the imperfections. As an indicator, clearly visible blue dots appear on the light yellow pads. At these locations, the protective passive layer does not exist on the steel surface. The KorroPad procedure is a non- destructive testing method. Thus, pipes and containers can be tested for risk of corrosion before the installation in a chemical plant in the framework of quality management. Another advantage is the test is easy to use. Previous knowledge in the field of corrosion or electrochemistry is not required. Three pads are needed per test that are attached to the stainless-steel surface. The pads have the approximate size of a five Euro- cent coin. After 15 minutes, the pads are removed with a plastic spatula and put on a plastic carrier foil. For analysis and documentation purposes, the test result can be scanned or photographed. If a risk of corrosion is detected, the material
experts together with the operators discuss the next steps. The KorroPad testing method is surface-effective and suitable for use with all relevant stainless-steel types. This is shown in extensive field tests carried out by TÜV SÜD Chemie Service. Austenitic chrome-nickel-molybdenum steel types were tested. In all tests with heat tinting after welding, the KorroPad showed indications. Also, electrochemical cleaning/polishing with respective devices or mechanical processing (e.g. brushing the welding seams) partially showed indications. Obviously, the removal of heat tinting had not been carried out sufficiently. Thus, the passive layer couldn't reform completely. The KorroPad tests at TÜV SÜD Chemie Service were supported by local electrochemical measurements. It revealed that where the KorroPad tests showed indications, a lower pitting potential existed. At these locations, an increased risk of corrosion must be assumed. The KorroPad procedure can also be used for quality management. Thus, imperfections at the outer side of new longitudinally welded pipes could be proved beyond any doubt. The use of KorroPad testing offers a
self-help solution. At the beginning, material surfaces almost always look optically smooth. But do the components keep in practice what the optics and the material name promise? There are many decisive factors that influence the answer: How were the surfaces treated? How were the welding seams post-treated? The results of KorroPad tests give a fast answer to these questions. In many tests and practical applications, TÜV SÜD Chemie Service confirms the suitability of the quick test. The use of the KorroPad test allows the surface of stainless steel to be tested right after delivery and after processing. Craft businesses can thus protect themselves from cost-intensive warranty claims.
TÜV SÜD Chemie Service
www.tuev-sued.de/chemieservice
PROCESS & CONTROL | JUNE 2018 27
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