ABSTRACT

The entry of hydrogen induced by atmospheric corrosion and its effect on the mechanical properties of advanced high-strength steels have been investigated in view of the safe application of these materials in the automotive industry. High-strength steels are susceptible to hydrogen embrittlement, and atmospheric corrosion is one of the possible sources of hydrogen uptake. To induce the corrosion reaction, bare and scribed zinc coated high-strength steel samples with a tensile strength of 1 GPa were contaminated with sodium chloride and exposed to humid air or immersed in water electrolyte. The corrosion-induced formation and permeation of hydrogen through steel was detected on the other side of the samples by using a scanning Kelvin probe and an electric hydrogen resistance sensor. Changes in mechanical properties were followed using slow strain rate testing. In contrast to immersion conditions, atmospheric corrosion was shown to produce a quantity of hydrogen insufficient to cause hydrogen embrittlement. An observed drop in the elongation and tensile strength of the bare steel was attributed to the formation of stress concentrators and the reduction in the cross section of the samples. Although hydrogen uptake due to atmospheric corrosion cannot be fully neglected, the studied steel grade DP1000 is considered safe for standard automotive applications.

INTRODUCTION

Hydrogen embrittlement is a process that results in a decrease of the ductility of metals as a result of absorbed hydrogen. Advanced high-strength steels used in the automotive industry are materials that are considered prone to hydrogen embrittlement. Hydrogen can enter the material during steelmaking or processing steps, such as pickling, cleaning, phosphating, and electroplating. A recommended or even mandatory step after steel production is reducing the hydrogen content by baking at a temperature of about 200 °C [1]. Most of the absorbed hydrogen diffuses out to the atmosphere, while residual hydrogen is homogeneously redistributed in the bulk material.

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