Effect of cathodic polarisation on corrosion fatigue behaviour of ion nitrided AISI 4140 steel

Abstract

It is well known that controlled changes in the electrode potential of specimens strongly influence the behaviour of corrosion fatigue for alloys in aqueous media. In other words, the effect of polarisation on the corrosion fatigue performance of steel can be handled depending on the magnitude of applied potential. In the present work, the effect of applied polarisation potential from −750 mV (SCE) to −1500 mV (SCE) on the corrosion fatigue performance of AISI 4140 steel has been investigated by conducting a series of rotary bending, corrosion fatigue tests at 95 Hz, in aerated 3% NaCl aqueous solution. Hourglass shaped, 4 mm diameter fatigue specimens were ion nitrided at 748 K for 1 h prior to tests. It was found that corrosion fatigue life was enhanced by increasing the applied polarisation potential from −750 mV (passive range) to −1080 mV (cathodic range). Extremely rare and shallow pit formation was detected at the surface of specimens under the influence of −1080 mV potential, and the corrosion fatigue strength was increased by 2.6 times the free corrosion fatigue strength based on a fatigue life of 10 6 cycles. An improvement of 93% in fatigue strength could be attained by cathodic protection referring to the data in air for a life of 3×10 6 cycles. An attempt was made to quantify the progress in corrosion fatigue performance by using the parameter of protection ratio, PR. Over-cathodic protection at −1500 mV adversely affected the corrosion fatigue performance of ion nitrided steel due to hydrogen embrittlement, which yields an easy intergranular type of crack initiation and propagation, particularly in the case of the ion nitrided region.