Stress corrosion cracking (SCC) of uncoated and AlSi-coated hot forming steel grades of 1500 and 1900 MPa tensile strength is investigated under different corrosive conditions using the VDA 238-201 step load test and the VDA 238-202 constant load test. Hydrogen charging was realized by cathodic polarization with an attached zinc coated sheet or by acid solutions.
The 1900 MPa grades show a significantly higher susceptibility to SCC compared to the well established 1500 MPa hot forming steel 22MnB5. In general, e-coated CR1900T specimen with a controlled damage of the paint surface by stone chipping show higher failure loads and thus a reduced SCC risk compared to bare specimen. This positive effect of e-coating is even stronger for AlSi-coated material.
Since many of the 1900 MPa specimen fail during the first 24 h / 50% load step, a modified step load test sequence with a lower minimum stress level and a longer step duration is recommended for 1900 MPa steel grades. To simplify test preparation, specimen without attached zinc coated sheet are tested in an aqueous acid solution with pH1. These conditions lead to similar failure loads than under cathodic polarization with attached zinc coated sheet in neutral 5% NaCl solution.
As a conclusion, due to their relatively high susceptibility to stress corrosion cracking, the 1900 MPa hot forming grades are not recommended under wet corrosive conditions in automotive body applications, especially when they are in contact with zinc coated parts.
Stress corrosion cracking, hydrogen embrittlement, hot forming steel