Question: For some months now, we have been struggling with a series of complaints for which we cannot find an explanation. For a good customer, we process very different parts made of equally diverse metals and alloys. One series of articles is made of stainless steel, whereby we also receive different alloys. These are simply chemically pre-treated to achieve a clean surface. Other parts made of ordinary mild steel are thickly galvanized by us. The two metals are welded together by the customer. The problem is that sooner or later cracks appear in the stainless steel. The customer claims that it is either due to the galvanizing or the chemical pre-treatment. We, on the other hand, are of the opinion that it must be a material defect, as we are not aware of such defects and extensive investigations of our processes have not yielded any results. As the customer is now threatening to give everything we coat for him to other electroplating companies, but we have no clues as to what the problem could be, we urgently need help.
Answer: In general, it is rarely a good idea to join metals with different potentials, as contact corrosion can occur at some point [1].
Contact corrosion
The following conditions must be met for contact corrosion to occur:
- Potential difference between the contact metals
- Electrical contact between the metals
- an electrolyte must wet both metals
- at least one of the materials must be in an active state
If the potential difference is below 50 mV, galvanic effects can usually be neglected. However, the electrode potentials of interest do not result from the theoretically calculated normal potentials of the elements. Since this voltage series refers to 1 molar metal ion solutions at pH = 0, which are not present in real corrosion processes, there are often considerable deviations from these potentials in practice. The extent of damage that can occur under the conditions of contact corrosion does not depend on the potential difference between the contacted metals, but on the actual corrosion current that flows when the metals are short-circuited. This current is determined on the one hand by the polarization effects at the local anode or cathode, but also by the surface ratio of the more electronegative to the more electropositive metal. If the anode area is small compared to the cathode, the cathodic reaction usually determines the metal dissolution at the anode, i.e. a very high corrosion current density is imposed on the anode, resulting in a high removal or depth effect. The above-mentioned surface effects also become clear with clad metals, where the above-mentioned surface ratios also have an effect on cut edges, holes or layer defects and can either lead to increased dissolution of the anode surfaces or to so-called remote protection. In the latter case, small anodic areas are cathodically protected. However, this requires good conductivity of the electrolyte. When welding different types of materials, care must be taken to ensure that a large anode is contrasted with the smallest possible cathode. Contact corrosion can often be avoided by using common coatings or overlays. One method is to apply a sacrificial metal. This is a metal that is even less noble than the anodic component compared to both materials of the metal pairing. If contact corrosion cannot be excluded by the measures mentioned, at least anodically effective components should be oversized (corrosion allowance) or easily replaceable, but small anodic surfaces should never be located opposite large cathode surfaces. Even if the construction is further coated later, it is quite possible, depending on the environment, that the problem of contact corrosion has been delayed but not eliminated. Although none of this has anything to do with the cracks, it may indicate initial errors in thinking when selecting the materials.
Stress corrosion cracking
What you describe as a defect indicates stress corrosion cracking. This can occur not only in metals [2]. In stress corrosion cracking, cracks occur either transcrystalline, i.e. through the crystallites, or intercrystalline, i.e. between the crystallites along the grain boundaries. The prerequisites for damage caused by stress corrosion cracking are the simultaneous effect of mechanical tensile stresses (positive normal stresses) and a specific, corrosive medium. Crack propagation occurs perpendicular to the tensile stress. The defect you describe is mainly known to occur in austenitic chromium-nickel steels that are welded to hot-dip galvanized steels. These chromium-nickel steels must not come into contact with metallic zinc, e.g. with zinc coatings or zinc dust coating, during heating or welding. At the welding point, zinc diffuses intergranularly into the steel and triggers intergranular stress corrosion cracking in the zones subjected to tensile stress by the thermal stresses. Similar to zinc, indium, lithium, cadmium, antimony and copper as pure melts or in alloys also lead to intercrystalline stress corrosion cracking in sensitive steels. While no damage occurs in pure lead melts below 800 °C, small amounts of zinc, antimony or copper trigger stress cracks even below 450 °C [3]. The tricky thing about stress corrosion cracking is that it occurs spontaneously on the outside and therefore cannot be seen coming. In our opinion, it is therefore neither a material defect nor an electroplating defect, but a design defect. You should discuss this with your customer and work out a better solution with him that also avoids the risk of contact corrosion.
Literature
[1] Online course "Fundamentals of corrosion ": https://www.galvanotechnik-for-you.de/uebersicht-kurse/grundlagen-der-korrosion/
[2] Article "Burst aquadome: was stress corrosion cracking the cause?" Galvanotechnik 02/2023 https://www.leuze-verlag.de/fachzeitschriften/galvanotechnik/item/6117-geplatzter-aquadom-war-spannungsrisskorrosion-die-ursache
[3] Prof. Dr.-Ing. Karl-Helmut Tostmann - Corrosion protection in theory and practice; 1st edition 2017; Eugen G. Leuze Verlag GmbH & Co. KG Bad Saulgau
