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    A brit­tle topic

    Functional components of trucks and trailers are constantly subjected to differing levels of stress: stone impact, dirt, frost, high operating temperatures or mechanical and dynamic influences. It is therefore all the more fatal if these components fail - for example due to hydrogen embrittlement.

    This is a phenomenon that appears to occur “out of the blue”: so-called hydrogen-induced stress corrosion cracking results in sudden failure in high-tensile components in particular. Construction parts but also connecting elements can break at any time. In trucks and trailers this sudden loss means downtime, delayed delivery and increased costs.

    Interaction of various causes

    The risk of hydrogen embrittlement only exists for high-tensile steel above a strength of > 1,000 N/mm², it is caused by hydrogen atoms diffusing into the steel. There are three possible factors influencing this:

    1. structural defects, inclusions, impurities or mechanical stresses in making the steel,
    2. defects in the manufacture of components from steel via measures such as forming, hardening or thermal processing,
    3. coating of the component. In pickling or cleaning procedures and the galvanic coating of ferritic steel parts atomic hydrogen may arise in the process bath, which can diffuse into the steel surface.

    However, it is most often the critical interaction of various influencing factors that ultimately results in failure of a component without any prior damage being noted.

    A gradual process

    The atomic hydrogen migrates within the steel to the grain boundaries and to defective areas – such as interior and exterior notches, punched edges or burrs. Here it enriches itself, weakening the metallic compound until a microscopically fine crack is created. Although this eases tension in this zone, at the tip of the crack new tension concentrations arise, which in turn attract more atomic hydrogen, weaken and crack. Ultimately, the remaining cross-section can no longer bear the external tensile load and a delayed brittle fracture occurs.

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    Intercrystalline hydrogen cracking (SEM image 10µm, source GSI NL SLV Duisburg)
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    Transcrystalline hydrogen cracking (SEM image 10µm, source: GSI NL SLV Duisburg)

    DIN 50969-1 describes how the aforementioned influencing factors can be reduced via the constructive design of the component, material and manufacturing technology measures and by reducing tensile residual stress. In coating, too, an attempt can be made to minimise hydrogen absorption via corresponding process control - for example where pretreatment does not pickle, but instead blasts or degreases using alkaline substances. Hydrogen can also be diffused again by tempering. However, this is dependent on the structure of the galvanic coating and is time and therefore cost intensive.

    Zinc flake as “relaxed” alternative

    The best solution is therefore to use a coating system in which no hydrogen is offered in the process. Non-electrolytically-applied zinc flake coating is therefore a good choice when faced with the challenge of protecting a high-tensile steel component from corrosion. This is a “lacquer” comprising numerous small flakes to protect components of various kinds against corrosion. The sacrificial behaviour of the ignoble zinc actively protects it against environmental influences. This is known as cathodic corrosion protection.

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    SEM image of a zinc flake coating with a thickness of 20µm.

    Zinc flake coatings typically contain a combination of zinc and aluminium flakes (in accordance with DIN EN ISO 10683 or DIN EN 13858), linked via an inorganic matrix. The protective coats are usually applied at a thickness of between 8 and 12 µm, enabling very high corrosion durability in salt spray testing.

    Duration of alt spray test (without red rust)Reference layer thikness of the coating system*
    > 600 h6 µm
    > 720 h8 µm
    > 960 h10 µm
    *The reference layer thickness includes both base- and topcoat - depending on the composition of the coating system

     

    Flake-like zinc particles, combined by a binding matrix, cross-link on the component. This may already occur at room temperature; however, most products are annealed at temperatures of 180–220°C. There is no more gentle way of applying cathodic corrosion protection. Depending on the component, different application forms may be advisable, such as the dip-spin process for screws, or spray application for larger parts.

    No real cost factor

    Corrosion damage to heavily stressed construction elements or fastening systems of trucks and trailers can result in failure, leading to damage and follow-on costs of a level that bears no comparison with the cost of the coating system employed. The use of zinc flake system enables high-performance cathodic corrosion protection here – without the risk of process-related hydrogen embrittlement during the application process and with the prospect of longer value retention and the reliable use of expensive capital goods.