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Hot-dip galvanizing has few major effects on the properties or performance of mild steels that are made by traditional techniques, except in improving durability and offering a low level of stress relief from fabrication stresses caused when the item is heated to the galvanizing temperature.
Sources of Embrittlement
Certain types of steels and fabrication methods that involve extreme cold-working of the steel before galvanizing can give rise to embrittlement issues that affect the performance of the item.
Types of Embrittlement
There are three important types of steel embrittlement that can be related to the hot-dip galvanizing process: liquid metal embrittlement, hydrogen embrittlement and strain age embrittlement.
Liquid Metal Embrittlement
Liquid metal embrittlement is brought about by the influence of the molten metal (zinc in the case of galvanizing) on vulnerable steels.
The most common liquid metal embrittlement issues related to hot-dip galvanizing are with stainless steel. For this reason, mounting stainless steel fittings to mild steel items before galvanizing should be avoided as the molten zinc may affect the mechanical properties of the stainless steel.
The diffusion of atomic hydrogen into the structure of susceptible metals like high-strength steel can gravely affect some mechanical properties. Sustained tensile stress can therefore result in failure. Losses of tensile or torsional ductility can be detected through dynamic and static laboratory testing.
Hydrogen embrittlement occurs due to the presence of hydrogen atoms inside the crystal lattice structure of an alloy or metal. During galvanization, hydrogen may be taken up in the steel at the time of the pickling process due to contact with the hydrogen ions found in the hydrochloric acid.
The metals most vulnerable to hydrogen embrittlement are steels with a tensile strength in the order of 1000 MPa or higher, or with an equivalent surface hardness of 30 Rockwell C or higher.
Hydrochloric acid is used at ambient temperatures in hot-dip galvanizing processes all through Australia, almost entirely for pickling before galvanizing. Acid concentration is normally 10%–15% HCl.
Most hot-dip galvanized steels are in the range of 200–450 MPa, hence do not experience hydrogen embrittlement issues. Steels with higher strength, like the quenched and tempered Bisalloy steels, are emerging in the structural area and should be given exclusive consideration before being hot-dip galvanized.
Avoiding Hydrogen Embrittlement
The condition to galvanize high-strength steels is extremely small compared to the volume of lower strength product that is regularly processed via galvanizing plants. It is possible to galvanize high-strength steels in an acceptable way by taking the essential precautions in the galvanizing process.
The suggested technique of processing high-strength steels for galvanizing is to avoid the acid pickling process and employ mechanical cleaning approaches for preparing the surface before hot-dip galvanizing.
Abrasive blast cleaning to Class 2 1/2 instantaneously before galvanizing will guarantee that the steel is sufficiently cleaned and an acceptable hot-dip galvanized coating will be developed.
Australian Standard AS 1214-1973 Appendix C asserts the following with regard to hydrogen embrittlement of high-strength bolts, which are the most commonly handled high-strength steels that require galvanization.
When extra protection is required (for example, for bolts of Grade 10.9 or higher cleaned by acid pickling), fasteners must be baked at a temperature of 200 °C + 10 °C for a time established based on the experience to be satisfactory (for guidance, a time of 30 minutes before galvanizing, or 4 hours right after galvanizing, might be adequate).
Strain Age Embrittlement
Strain aging is related to strain that is developed from plastic deformation, which is more commonly referred to as cold-working. Steel is an alloy of carbon and iron and consists of other alloying elements that endow it with specific performance characteristics.
Extreme cold-working of steel leads to the migration of carbon atoms in the iron crystals, and the segregation of these atoms at dislocations in the steel leads to a decrease in ductility.
The aging process is a function of time and temperature and takes place very slowly in ambient conditions but very quickly at the 450–460 °C temperatures of the galvanizing process. Extreme cold-working of steel can be brought about by hole punching in thicker sections, rebending or tight radius bending.
It must be noted that it is the heat of the process that is the reason for speeding up the strain aging of the steel, and not hot-dip galvanizing; therefore, strain age embrittlement can be initiated in any rigorously cold-worked steel by heating and the propensity to embrittlement by strain aging will always be present.
Avoiding Strain Age Embrittlement
In order to eliminate the risk of strain age embrittlement the following design criteria should be followed:
- Hot bend if bend radii below three times the section thickness are needed
- Use bend radii that are at least three times the section thickness
- Ream punched holes to eliminate rigorously cold-worked material from the surface before galvanizing
- Anneal at 650–815 °C before galvanizing