Influence of Strain on the Environmental Hydrogen-Assisted Cracking of a High-Strength Steel in Sodium Chloride Solution

Evidence is presented that will establish the role of mechanical strain in promoting surface absorption of hydrogen by rupturing surface films on AISI(1) 4340 steel in alkaline 3.5% sodium chloride solution. Such bare surface generation is shown to substantially influence the alloy’s hydrogen cracking susceptibility. The effect of strain-enhanced absorption is reported for smooth, tapered, and notched tensile specimens under slowly straining conditions and is also shown to explain hydrogen-assisted cracking behavior for double cantilever beam fracture mechanics specimens with pre-existing fatigue cracks. The interrelationship between slow continuous straining at a given strain rate to produce a minimum critical strain for film rupture and the associated time will be shown to be a critical factor for this system in all specimen types. Enhancement of hydrogen absorption per unit area of bare surface created with film removal, as determined by Devanathan-Stachurski hydrogen permeation measurements, explain the effect. Additional evidence based upon interpretation of permeation distances and the dimensions of the crack tip plastic zone suggests that the rate determining hydrogen transport step for hydrogen-assisted cracking of AISI 4340 steel in alkaline seawater is hydrogen absorption.