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Rapid corrosion test for stainless steels

05 November 2018

Stainless-steel piping and tanks have a passive layer that protects them from corrosion. If this layer is incompletely formed or damaged, it can pose problems. Dr Helga Leonhard and Jens Lehmann discuss a rapid test that can be used to check the condition of the passive layer.

KorroPad test can indicate a defect in the passive layer. (Image courtesy: BAM)
KorroPad test can indicate a defect in the passive layer. (Image courtesy: BAM)

A flawless passive layer on the material surface of stainless steel is essential. However, it is not always easy to ascertain whether the passive layer has fully formed and how to positively influence the quality of the natural surface protection during processing. 

While they provide clarity on corrosion behaviour, traditional test methods, such as electrochemical measurements and salt-spray tests, they do require special equipment and expert knowledge so can be costly. Further, they are destructive test methods and can take hours or even days to gain a result. This makes such test unfeasible from a financial and practical perspective for many processors. 

The KorroPad rapid test method, developed by the Bundesanstalt für Materialforschung und -prüfung (BAM), and verified by TÜV SÜD Chemie Service, changes this situation.

In order to select the right materials, plant engineering routinely relies on electrochemical measurement methods, such as determination of pitting potential by plotting current density-potential-curves in measurement cells in a lab or at individual locations on the component itself. The objective was to compare electrochemical measurements with the KorroPad method. If the KorroPad method proved suitable, processors and users of stainless steels would be provided with an easy-to-use on-site method. 

A shining, clean stainless steel surface is not necessarily an indicator that the right type of piping or tank material has been selected. Faults in, or inadequate formation of, the passive layer are not visible to the naked eye. In the chemical industry in particular, material surfaces have to withstand extreme conditions. During everyday production operations they are exposed to acids, corrosive gases and other aggressive fluids. Defects in the passive layer can lead to dissolution of materials and pitting corrosion, and ultimately causes failure. For this reason verification of the corrosion resistance of materials is important.
 
Passive layer 
Plant engineering mostly uses chromium-nickel-molybdenum steel, with chromium being the most primarily important element for corrosion resistance. The presence of humidity and oxygen causes passivation, resulting in the formation of a chromium oxide layer on the stainless steel surface. The requirement for passivation is a chromium content of at least 10.5%. The chromium oxide layer acts as a passive layer. However, it is only a few atom layers thin, is invisible and highly sensitive. Faulty or unfavourable conditions during production, processing, transport or storage can affect full formation of the passive layer and this will affect surface protection. The passive layer can regenerate in the presence of a sufficient amount of oxygen and humidity. Ultra-clean polished metallic surfaces also favour repassivation.

If the passive layer is incomplete, divalent ferrous ions are released from the material at the faults in the protective layer. The gel-like KorroPads are saturated with water containing small amounts of sodium chloride and a ferrous-ion indicator. The indicator is potassium hexacyanoferrate (III), which is yellow to transparent in aqueous solution and spontaneously changes to Prussian blue upon contact with the ferrous ions released from the material. Highly visible blue spots appear on the light-yellow pads, indicating the locations at which the protective passive layer is missing on the stainless steel surface or has not fully formed. 

Non-destructive test method
The KorroPad procedure is a non-destructive testing method. It can be used to verify the corrosion resistance of piping and tank components even before they are installed in a process plant.

Indications of corrosion and requirements for surface corrosion resistance (Image courtesy: BAM)
Indications of corrosion and requirements for surface corrosion resistance (Image courtesy: BAM)

The test is easy to use and does not require any previous or expert knowledge in the fields of corrosion or electrochemistry. The KorroPads are roughly the size of a five Euro cent coin. The surface to be tested needs to be cleaned with acetone or alcohol before the pads are placed on the surface and pressed down. The test only takes 15 minutes. When this period has passed, the pads are removed using a plastic spatula and placed on a plastic carrier film. Scanning or photographing the test result is helpful for evaluation and documentation. If the KorroPads reveal a risk of corrosion, the material experts will consult with the plant operator and plan the next steps.
 
The test method is primarily surface-specific and can be used on all relevant stainless steel types. This was confirmed in comprehensive practice tests carried out on austenitic chromium-nickel-molybdenum steels at TÜV SÜD Chemie Service. Tests conducted on temper colours after welding also – without exception – produced indications in KorroPads. The testers further observed that electrochemical cleaning/polishing using devices designed for this purpose or mechanical treatments (such as brushing the weld seams) also sometimes resulted in indications. Obviously temper colours had not been sufficiently removed which hindered full re-formation of the passive layer. 

Parallel to KorroPad testing in practice, TÜV SÜD Chemie Service carried out local electrochemical measurements for comparison. The results showed that pitting corrosion potential was lower at the points where KorroPad testing resulted in indications, meaning that corrosion risk is higher at these locations.  This rapid test can also be used to verify that passivation has fully developed after grinding, etching and other cleaning steps, confirming that no problems need be expected during operation later on. It was also possible to prove that the rapid test is suitable for quality assurance. The method clearly identified faults on the exterior of new pipes with longitudinal welds.

On the safe side 
TÜV SÜD Chemie Service confirmed the suitability of the KorroPad method in many tests and applications in practice. The method can be used to verify the corrosion resistance of steels both as delivered and after processing. Given this, it provides clarity and reliability in corrosion protection right from the outset. 

The method can also be used to characterise many process-related factors influencing surface quality and material. Thanks to its rapid application and the easy evaluation of test results, the method enables both positive and negative changes in passive-layer stability to be identified immediately. This allows users to respond quickly, eliminate critical influences and initiate improvements to in-house processes. Other potential areas where KorroPads may be used include incoming and outgoing goods inspection.

Dr Helga Leonhard  is a test engineer at TÜV SÜD Chemie Service. Jens Lehmann is a research associates at  Bundesanstalt für Materialforschung und-prüfung (BAM).


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