1. General Provisions
Hydrogen operation in metals depends on the nature of the metal, the degree of its purity, the presence of alloying elements, the distribution of pressure, the nature of defects and other factors. It is known that hydrogen diffusing into the crystal grating of a metal can interact with different defects. Hydrogen accumulation in metal defects causes significant degradation of the material.
Hydrogen embrittlement becomes apparent to the maximum extent upon hydrogenation of high-strength and hardened steels used for manufacture of important and high-rate fasteners which need to provide maximum operational reliability. The metal may contain hydrogen in different forms(including the dissolved (atomic), extracted and different bound forms). Carbon steels incorporate the interaction of hydrogen and carbon, with the formation of hydrocarbon compounds, which may lead to irreversible changes in the structure. The diversity of modes of hydrogen interaction with metals explains the inconsistency of data related to the dependence of the mechanical properties of high-strength steels on the total hydrogen content.
Hydrogen embrittlement of high-strength steels may be caused by a small amount of hydrogen.
Hydrogenation of low-carbon steels used for manufacture of most fasteners does not lead to embrittlement.
Hydrogenation of steel products under electrochemical processes occurs to some extent at all stages of fastener galvanizing. The decisive contribution in hydrogenation of steel is made by the galvanizing itself, however the influence of preliminary operations should also be taken into account when considering the issues of hydrogen embrittlement, therefore cathodic degreasing of high-strength steels is not applied.
To prevent hydrogen embrittlement, fasteners are exposed to thermal dehydrogenation after electrogalvanizing.
The process heating of parts until complete removal of hydrogen is long-term and energy-consuming. For fasteners with bright coatings it may be impossible to achieve complete removal of hydrogen even in the instance of continuous dehydrogenation because of their extremely low hydrogen penetration.
In the machine-building industry electrogalvanizing is acceptable for steels with the strength of up to 1,000 N/mm2if corresponding activities to reduce hydrogen embrittlement are carried out. At the same time, electrogalvanizing of highly loaded fasteners made of steel with the strength of 1,000 N/mm2 and over which require increased reliability is prohibited.
Numerous tests show that electrogalvanizing causes reduction in the relative elongation value by up to 80% and the number of cycles to fracture of the sample by up to 60% in comparison with uncoated samples. Mechanical galvanizing does not lead to degradation of mechanical properties of fasteners, and relative elongation is even slightly increased.
Heating of coated fasteners to 250°Сhas a positive impact on the reduction of hydrogen embrittlement of high-strength steels but has a negative impact on the coating itself. Dehydrogenation reduces corrosion resistance of chromate films and the appearance of the coating deteriorates.
2. Hydrogenation Mechanisms and Influencing Factors
In electrochemical processes hydrogen is extracted on the cathode, i.e. on the surface of the coated part. During galvanizing, the reactions of hydrogen extraction and metal ions discharge are always interrelated and occur simultaneously.
The depth of hydrogen penetration into the steel base may reach 100 ηm.
Organic impurities influence the dynamics of subsequent thermal dehydrogenation of the coating and the base changing hydrogen permeability of zinc and complicate hydrogen removal from steel. Bright coatings usually have many more organic impurities than frosted coatings, and their degassing is complicated. Heating of frosted coatings leads to almost complete removal of hydrogen, while with parts with bright zinc complete removal of hydrogen is not achieved even upon continuous heating for 10-20 hours.
Hydrogen embrittlement of high-strength steels, as a negative consequence of metals hydrogenation, depends on all the factors which determine hydrogenation of the “steel-coating” system. However, steels themselves show different tendencies to embrittlement depending on their chemical composition, the carbon content, alloying additives and impurities. Electrogalvanizing cannot be considered the only reason for hydrogen embrittlement of high-strength steels. The primary importance is given to metal structure, the modes and methods of treatment and also the surface condition of parts delivered for galvanizing.
Engineering standards provide special tests for the tendency to hydrogen embrittlement; the results of these tests are used for the development of redemptory actions focused on reduction of the negative impact from hydrogenation. A structural line is made in descending order in terms of the tendency to hydrogen embrittlement. Combination of the appropriate metal structure and technology of coating application, including the type of electrolyte, provides a significant reduction in the risk associated with brittle fracture of the product.
3. Some Measures for Reduction ofHydrogen Embrittlement