High strength fasteners
Fasteners with a strength level of 10.9 (tensile strength of 1000Mpa) or higher are generally considered high-strength fasteners. At present, grade 10.9 and 12.9 bolts are widely used in automobiles, for example, high-strength fasteners are generally used in the fastening connection positions on the car chassis.
Because the pre-treatment of electroplated fasteners requires an acid washing process, the purpose of acid washing is to remove the oxide scale or rust on the surface of the product through sulfuric acid or hydrochloric acid, and increase the surface activity of the product. Due to the presence of acid, the presence of hydrogen ions is inevitable. Therefore, there is a risk of hydrogen embrittlement in the parts, and usually the higher the strength of the parts, the greater the risk of hydrogen embrittlement. To avoid hydrogen embrittlement, acid buffer should be added during the pickling process to reduce the impact of acid on the product. In addition, after electroplating and before passivation, the product should be subjected to dehydrogenation treatment to reduce the probability of hydrogen embrittlement. Therefore, it is not recommended to use electroplating surface treatment for bolts of grade 10.9 and above.
The method for removing rust and oxide scale on the surface of parts with zinc aluminum coating is shot blasting. Shot blasting is a physical surface treatment method that does not pose a risk of hydrogen embrittlement. Therefore, it is very suitable for surface treatment of high-strength bolts (≥ grade 10.9).