Can I Hot-Dip Galvanize A490 Bolts?

The term A490 Bolts refers to high strength bolts covered by the standard ASTM A490, Standard Specification for Structural Bolts, Alloy Steel, Heat Treated, 150 ksi Minimum Tensile Strength. A490 covers heavy hex structural bolts (tensile strength of 150 to 173 ksi)  and currently, ASTM Standards do not allow hot-dip galvanizing of A490 bolts for corrosion protection. Citing a concern of hydrogen embrittlement, A490 only allows a type of zinc/aluminum coating to be used on high-strength bolts.

A recent study conducted by AGA Technical Director, Thomas Langill, Ph.D., and published in the April 2013 issue of Fastener Technology International demonstrates the feasibility of hot-dip galvanizing A490 bolts without a danger of hydrogen embrittlement. With failures due to hydrogen embrittlement occurring hours, weeks, months, or even up to a year after the galvanizing process, the cause of the problem has been shown to originate from the cleaning step of the hot-dip galvanizing process. During the process, the heating of the steel causes some of the atomic hydrogen absorbed from the pickling acid to migrate into trap sites within the steel matrix. Once in the trap site, the hydrogen atoms can cause a premature failure of the steel fastener when installing in the field. Therefore, removing the source of hydrogen atoms (pickling acids) from the galvanizing process essentially eliminates the concern for hydrogen embrittlement failure.

There is a modified galvanizing process approved in Europe and being successfully used by European galvanizers to reduce high-strength steels susceptibility to hydrogen embrittlement by reducing its exposure to hydrogen atoms. The time the steel spends immersed in the pickling acids must be eliminated as this is the only source of hydrogen atoms in the galvanizing process. 

The process is modified after the degreasing stage and consists of:

Mechanical surface preparation of the steel (blasting or wheel abrading)

Thomas Langills aforementioned study tested several bolts galvanized with this modified process for compliance with ASTM and other standards. This study followed the Industrial Fasteners Institutes test procedure IFI 144, Test Evaluation Procedures for Coating Qualification Intended for use on High-Strength Structural Bolts. In regards to this study, the tests for hydrogen embrittlement, namely  ASTM F606 and F1624, were the most important. High strength bolts mechanically cleaned and galvanized according to the modified galvanizing process cited above passed both of these tests. The bolts were also subject to tests standard to the hot-dip galvanizing industry to determine paintability, corrosion protection, coating thickness, and adhesion. Results of these tests were typical for hot-dip galvanizing and showed no adverse effect on the coating from the modified process. The galvanized coating on the high-strength bolts met the requirements of ASTM A153.

It must be emphasized again ASTM A490 currently does not permit the use of hot-dip galvanizing corrosion protection for high strength (>150 ksi tensile strength) bolts. While the results of this study are indeed promising and indicate a change to the standards may be made in the future, they are not to be used as standard procedure. If a customer wants to hot-dip galvanize high strength bolts, results from this study can be used to reach an agreement. However, the customer must be made aware this modified process is not currently in compliance with ASTM standards.

Many galvanizers helve successfully galvanized A490 bolts at times. However, the research work completed in the late 705 and early 80s showed, failure rates of up to 25% for hot-dip galvanized A490 bolts. ASTM has now excluded all metallic coatings from use on A490 bolts, If your customer insists on having you do the work, it is suggested that you send him a letter informing him of the ASTM action and asking him to authorize your work in writing, AGA can assist you in this matter.