International Journal of Fatigue 24 (2002) 537–543 www.elsevier.com/locate/ijfatigue Effect of cathodic polarisation on corrosion fatigue behaviour of ion nitrided AISI 4140 steel Kenan Genel a , Mehmet Demirkol b,* , Mustafa U ¨ rgen c a Mechanical Engineering Department, Sakarya University, Adapazari 54040, Turkey b Faculty of Mechanical Engineering, Istanbul Technical University, Gu ¨mu ¨s ¸suyu 80191, Istanbul, Turkey c Faculty of Chemical and Metallurgical Engineering, Istanbul Technical University, Ayazag ˇa 80626, Istanbul, Turkey Received 12 April 2001; received in revised form 8 August 2001; accepted 9 August 2001 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.  2002 Elsevier Science Ltd. All rights reserved. Keywords: Ion-nitriding; Corrosion fatigue; Cathodic polarisation; Protection ratio PR; Low alloy steel 1. Introduction Thermo-chemical surface treatments are recognised as one of the most important applications in enhancing the fatigue performance of machine components. Ion or plasma nitriding is widely used to increase fatigue and corrosion fatigue strength of Fe based alloys [1–3]. The studies have shown that the corrosion fatigue cracking in ion nitrided specimens is basically nucleated at the root of corrosion pits [3–5]. It is generally agreed upon that fatigue crack initiation under the influence of a cor- rosive environment is closely related to the combination of mechanical and electro-chemical effects in cyclic * Corresponding author. Tel.: +90-212-293-13-00x2582; fax: +90- 212-245-07-95. E-mail address: demirkol@mkn.itu.edu.tr (M. Demirkol). 0142-1123/02/$ - see front matter  2002 Elsevier Science Ltd. All rights reserved. PII:S0142-1123(01)00114-1 stressing, and any reduction in the effectiveness of one of these factors yields an improvement in corrosion fatigue resistance [5–8]. It has been reported that ion nitriding has beneficial effects particularly on the cor- rosion fatigue performance of low alloy steels and some improvements of up to 70% could be reached with respect to the performance of quenched and tempered steels [4,5]. The application of cathodic polarisation also suppresses the metal dissolution and pit formation, and thus reduces the possibility of corrosion fatigue crack nucleation at the surface of specimens [9–12]. Cathodic polarisation potential has a significant influence on cor- rosion fatigue performance of high strength steels, and best results could only be reached in certain critical ranges of applied polarisation potentials. It has also been reported that the risk of easy cracking due to hydrogen embrittlement in excess polarisation, and insufficient corrosion protection of surfaces in the case of low polar-