Corrosion behaviour of nitrided low alloy steel in chloride solution

The corrosion behaviour of low alloy steel type AISI 4130 (before and after nitriding) and austenitic stainless steel type AISI 304L were studied in tap water +3.5 per cent NaCl. A liquid nitriding process had been applied on the low alloy steel.

The tests that were carried out in this study were anodic polarization, rotating bending fatigue and axial fatigue using compact tension (CT). For determining the corrosion potential and pitting potential (breakdown potential) for the alloys, anodic polarization curves were established using the potentiodynamic technique. Rotating bending fatigue tests were used to calculate the fatigue strength and damage ratio. Using linear elastic fracture mechanics, the CT specimens were prepared for determining the threshold stress intensity factor, fatigue crack growth rate and fracture toughness in air and in the solution.

The results showed that nitrided specimens showed higher fatigue strength in air compared to stainless steel. However, the corrosion fatigue limit for both these samples were approximately equal, while this limit for non‐nitrided sample was less. Moreover, the non‐nitrided steel had lower corrosion and pitting potentials than did the stainless steel. In addition, the CT tests showed that the nitrided specimens had a lower resistance to crack initiation in air and the solution compared to the non‐nitrided sample and the stainless steel.

These results can be attributed to the chemical and mechanical behaviour of the nitrided layer constituents, mainly FeN and CrN, which were recognized by X‐ray diffraction. Since, these components consist of very hard particles, they act to increase the hardness and fatigue limit. Moreover, due to the low conductivity of these nitrides, the corrosion and pitting potential of the nitrided steel becomes very high. However, the high breakdown potential does not help to increase the corrosion fatigue or damage ratio values due to the porous nature of the nitrided layer.

Although the nitrided steel had very high fatigue strength and pitting potential, this did not reflect in its corrosion fatigue and/or damage ratio improvement because of its surface roughness and the porous nature of the nitrided layer.