May 9 2002
A.S.U. Photo / Shutterstock
Hot-dip galvanizing can have some major impact on the performance or properties of mild steels that are produced using traditional methods. This process, however, enhances durability and provides a low level of stress relief from fabrication stresses caused when the product is heated to the galvanizing temperature.
Sources of Embrittlement
Some types of steel and fabrication techniques that require extreme cold-working of the steel before galvanizing can lead to embrittlement problems that impact the performance of the product.
Types of Embrittlement
Three major types of steel embrittlement can be associated with the hot-dip galvanizing process. These include hydrogen embrittlement, liquid metal embrittlement, and strain age embrittlement.
Liquid Metal Embrittlement
Liquid metal embrittlement occurs because of the impact of the molten metal (zinc in the case of galvanizing) on vulnerable steels.
Stainless steel experiences the most common liquid metal embrittlement problems associated with hot-dip galvanizing. Therefore, mounting stainless steel fittings to mild steel items prior to galvanizing should be avoided because the molten zinc may impact the mechanical properties of the stainless steel.
The diffusion of atomic hydrogen into the structure of vulnerable metals like high-strength steel can seriously have an impact on certain mechanical properties. Therefore, sustained tensile stress can result in failure. Losses of torsional or tensile ductility can be identified through static and dynamic lab testing.
Hydrogen embrittlement takes place because of the presence of hydrogen atoms within the crystal lattice structure of a metal or alloy. During galvanization, hydrogen may be absorbed in the steel during the course of the pickling process, as a result of contact with the hydrogen ions present in the hydrochloric acid.
Steels having a tensile strength of 1000 MPa or more, or with a similar surface hardness of 30 Rockwell C or more, are the kinds of metals that are most vulnerable to hydrogen embrittlement.
Hydrochloric acid (HCl) is used at ambient temperatures in hot-dip galvanizing processes throughout Australia, almost entirely for pickling prior to galvanizing. The concentration of acid is generally 10%–15% HCl.
The majority of hot-dip galvanized steels are in the range of 200–450 MPa, and hence do not undergo hydrogen embrittlement problems. Steels having more strength, like the tempered and quenched Bisalloy steels, are evolving in the structural area and must be given special consideration before being hot-dip galvanized.
Avoiding Hydrogen Embrittlement
The galvanizing condition of high-strength steels is very limited compared to the volume of lower strength product that is routinely processed through galvanizing plants. It is viable to galvanize high-strength steels in a satisfactory way by taking important precautions in the galvanizing process.
The recommended method of processing high-strength steels for galvanizing is to prevent the acid pickling process, and apply mechanical cleaning approaches for setting the surface before hot-dip galvanizing.
Abrasive blast cleaning to Class 2 1/2 immediately before galvanizing, will ensure that the steel is adequately cleaned and a satisfactory hot-dip galvanized coating is subsequently formed.
Australian Standard AS 1214-1973 Appendix C emphasizes the following with reference to hydrogen embrittlement of high-strength bolts. These bolts are the most commonly controlled high-strength steels that necessitate galvanization.
When additional protection is necessary (for instance, for bolts of Grade 10.9 or further cleaned by acid pickling), fasteners should be baked at a temperature of 200 °C + 10 °C for a period of time set based on experience (for guidance, 30 minutes prior to galvanizing, or 4 hours immediately after galvanizing, might be sufficient).
Strain Age Embrittlement
Strain aging is associated with strain that is caused due to plastic deformation, which is more typically called cold-working. Steel is an alloy of iron and carbon, and contains other alloying elements that provide it with specific performance features.
Extreme cold-working of steel results in the migration of carbon atoms in the iron crystals, and the separation of these atoms at dislocations in the steel results in reduced ductility.
The aging process is a function of temperature and time, and occurs very gradually in ambient settings; however, it occurs very rapidly at 450 °C – 460 °C temperatures of the galvanizing process. Extreme cold-working of steel can be caused by hole punching in thicker sections, re-bending, or tight radius bending.
It must be kept in mind that it is the process heat that is responsible for accelerating the strain aging of the steel, and not hot-dip galvanizing. Thus, strain age embrittlement can be triggered in any carefully cold-worked steel by heating, and the propensity to embrittlement by strain aging will invariably be present.
Avoiding Strain Age Embrittlement
In order to remove the hazard of strain age embrittlement, the following design measures should be followed:
- Hot bend if the required bend radii are less than three times the section thickness
- Use bend radii that are a minimum of three times the section thickness
- Ream punched holes to remove rigorously cold-worked material from the surface prior to galvanizing
- Anneal at 650 °C–815 °C prior to galvanizing