Avoidance of hydrogen embrittlement

Application-related hydrogen-induced stress corrosion may cause sudden failure of components on machinery and agricultural equipment. In order to avoid expensive and time-consuming repair work, a suitable form of corrosion protection needs to be chosen.

Maintenance or repair work on agricultural machinery and equipment entails expensive delays or machine downtime. One possible cause, which appears to occur “out of the blue”, may be application-related hydrogen-induced stress corrosion cracking. It can even lead to sudden failure during the assembly of the subsequently highly-stressed components. Construction parts, including high-strength fasteners, break without a moment’s notice. For machines that need to be in dependable, non-stop use during periods of soil cultivation, manure spreading or harvesting this is an absolute disaster.

Interaction of various causes

The risk of hydrogen embrittlement af- fects high-tensile steels above strengths of > 1,000 N/mm. and is favoured by, for example, structural defects, inclusions, impurities or mechanical stresses in working the steel. Further influencing factors arise in the manufacture of components from steel via measures such as forming, hardening or thermal processing. The coating of the component can also have an influence. 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. Finally, hydrogen charging may occur via corrosion hydrogen, i.e. when using the components.
It is 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 to grain boundaries and areas of defect within the steel, where 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.

Intercrystalline hydrogen crack (SEM picture 10μm, li), Transcrystalline hydrogen crack (SEM picture 10μm, re), Source: GSI NL SLV Duisburg

DIN 50969-1 describes how the influencing factors of hydrogen-induced stress corrosion can be reduced via the constructive design of a 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 pre-treatment does not pickle, but instead blasts or degreases using alka- line substances. Hydrogen can also be diffused again by tempering. However, this is dependent on the structure of the coating and is time and therefore cost intensive.

Risk prevention through zinc flake

The best solution is therefore to use a coating system in which no hydrogen is offered in the process. Non-electrolyti- cally-applied zinc flake coating is consequently 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 be- haviour of the ignoble zinc actively protects it against environmental influences. This is known as cathodic corrosion protection.

Relation of layer thickness to salt spray resistance as per DIN EN ISO 10683 (the stated reference thicknesses are for orientation only)

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 system, which consists of a basecoat and a topcoat, requires a coat thickness of just 8 to 20 μm and enables very high durability in various corrosion protection tests. Flake-like zinc particles, combined by a binding matrix, cross-link on the component. This may already occur at room temperature; however, most products cross-link 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 or spray application.

No question of cost

Effective zinc flake systems not only offer high-performance cathodic corrosion protection, the low coat thickness also means that they are not more cost-intensive than less suitable, conventional thick-coat applications.