A delicate subject

Hydrogen-induced stress corrosion cracking can cause sudden system failures in the functional components of trucks and trailers.

It is a phenomenon that seems to occur "out of the blue": hydrogen-induced stress corrosion cracking. It leads to sudden failure, especially in high-tensile components. Structural components, but also connecting elements can break at any time. For trucks and trailers, such sudden failures mean downtime, delayed deliveries and increased costs.

An interplay of different causes

The risk of hydrogen embrittlement only affects high-tensile steel variants with a strength of > 1,000 N/mm² and it is caused by hydrogen atoms that diffuse into the steel. This is due, for example, to structural defects, inclusions, impurities or mechanical tensions in steel production. Further variables add to this after processing steps such as forming, hardening or heat treatment during the production of steel components. The third influencing factor is the coating of the component. During pickling or cleaning processes and the galvanic coating of ferritic steel parts, atomic hydrogen can form in the process bath and can diffuse into the steel surface. Usually it is the critical interaction of different influencing variables that ultimately leads to the failure of a component without any apparent pre-existing damage.

A creeping process

The atomic hydrogen migrates in the steel to the grain boundaries and to flaws – such as outer and inner notches, punched edges or burrs. It accumulates there and weakens the metal bond until a microscopically fine crack develops. This releases tension in that zone, however, new stress concentrations arise at the tip of the crack, which in turn attract atomic hydrogen and then weaken and crack. Ultimately, the remaining cross section can no longer bear the external tensile load and a delayed brittle fracture can occur. DIN 50969-1 describes how the variables influencing hydrogen-induced stress corrosion cracking can be reduced by the design of the component, by material and production engineering measures and by the reduction of residual tensile stresses.

It is also possible to try to minimise hydrogen uptake during coating by appropriate process control – for example, by not pickling but instead blasting or using alkaline degreasing during pretreatment. The hydrogen can also be made to effuse again by annealing. However, this depends on the structure of the galvanic layer and is time-consuming and therefore costly.

Zinc flake as a "relaxed" alternative

The best solution is therefore to opt for a coating system that uses no hydrogen at all. Thus, the non-electrolytically applied zinc flake coating is a good choice to deal with the challenge of reliably protecting a high-tensile steel component against corrosion. This is a "varnish" consisting of many small flakes that primarily protects various types of components against corrosion. Due to the sacrificial effect of the base zinc, it actively protects against environmental influences. This is called cathodic corrosion protection.

Zinc flake coatings usually contain a combination of zinc and aluminium flakes (in accordance with DIN EN ISO 10683 or DIN EN 13858), which are connected by an inorganic matrix. As a rule, the protective layers are applied in a layer thickness ranging between 8 and 12 μm, which achieves very high corrosion resistance times in salt spray tests.¹ Zinc particles arranged like flakes, which are connected by a binder matrix, cross-link on the component. This can be done at room temperature, but most products are usually baked at temperatures of 180–220 °C. There is no gentler way to apply cathodic corrosion protection. Depending on the component, different application modes are recommended – for screws, for example, the dip spin process is suitable, while for larger components spray application can be used.

Radio of layer thickness to salt spray test resistance

No real cost issue

In the event of failure, corrosion damage to highly stressed structural parts or connection systems on trucks and trailers can lead to damage and result in costs that greatly outstrip the costs of the coating system. With zinc flake systems, high-performance cathodic corrosion protection can be achieved without the risk of process-related hydrogen embrittlement during the application process and with the prospect of long-term value retention and trouble-free use of expensive capital goods.

Figures 1 and 2 show examples of two different types of hydrogen crack:

Fig. 1: Intercrystalline hydrogen crack (SEM picture 10μm, source GSI NL SLV Duisburg)

Fig. 2: Transcrystalline hydrogen crack (SEM picture 10μm, source GSI NL SLV Duisburg)