The role of passivating oxide films on the stainless steel in wearing action has been studied as a function of applied potential and chloride ion concentration using a modified pin-on-disc type wear machine. Also, The growth kinetics of the passivating oxide film of nickel were investigated in the aqueous solution, and a theoretical analysis of the growth kinetics of iron has been made. Moreover, the role of the transferred films of poly tetra fluoro ethylene(PTFE) composites containing fluorinated ethylene proylene(FEP) in the wearing action has been studied using the modified pin-on-disc type machine.
In chapter III, the influence of applied potential on the corrosive wear behaviour of type 304-L stainless steel in 1 N $H_2SO_4$ solution has been investigated. The corrosive wear rate increased with increasing applied potential up to 600$mV_{SCE}$, and then decreased. The corrosive wear rate $W_{CW}$ consists of three factors, viz., pure mechanical wear loss $W_{MECH}$, weight loss caused by the corrosion reaction $W_{DISS}$, and acceleratory contribution due to the presence of passivating oxide film $W_{ACC}$. The fraction of the acceleratory contribution increased with increasing applied potential up to 200$mV_{SCE}$, and then decreased. In order to investigate the role of passivating oxide film in the corrosive wear process, several examinations such as potentiokinetic measurement, inductively coupled plasma(ICP) spectroscopy, friction coefficient measurement, and scanning electron microscopic(SEM) measurement were carried out. From the results of such experiments, the variations of corrosive wear rate and acceleratory contribution associated with the applied potential are discussed in terms of competing effects of corrosion reaction and compositional change of passivating oxide film.
In chapter IV, results on the influence of chloride ion levels on the corrosive wear rate of 304-L stainless steel in 1 N $H_2SO_4$ solution are discussed and continue from chapter III. The corrosive wear rate was measured at two different applied potentials of 200 and 600$mV_{SCE}$. For applied potential of 200$mV_{SCE}$ the corrosive wear rates increased linearly with increasing chloride ion concentration from 0 to 0.4 mol $l^{-1}$. For 600$mV_{SCE}$ the corrosive wear rates showed the increase similar to those at the 200$mV_{SCE}$, below the critical chloride ion concentration necessary for the occurrence of pitting. In addition, potentiokinetic, inductively coupled plasma(ICP) spectroscopy, ac impedance, current decay transient, and scanning electron spectroscopic(SEM) measurements were carried out. It is suggested that the increase of corrosive wear rate in the presence of chloride ion is caused by the decreased current efficiency due to the presence of chloride ion for the formation of passivating oxide film.
In chapter V, the effects of applied potential and chloride ion on the repassivation kinetics of the passivating oxide film of pure nickel in 1.5N $Na_2SO_4$ solution of pH 2.8, have been studied using a new laboratory technique(abrading method) for obtaining a bare surface. The current transient experiments were carried out in the applied potential range of -200$mV_{SCE}$ and 600$mV_{SCE}$ in the solutions with(0.15 to 0.2 mol $l^{-1}$) and without chloride ions. The current decay profile was observed even in the active anodic dissolution region(-200$mV_{SCE}$). The current decay rate increased with increasing applied potential from precursor region(0 and 200$mV_{SCE}$) to passive region (400 and 600$mV_{SCE}$). As the chloride ion concentration increased at a given potential, for a short exposure time the current density increased, while for a long exposure time it decreased, below the critical chloride ion concentration for the occurrence of pitting. Also, the intersection time($t_i$) at which the same current density was achieved in the solutions with and without chloride ion, decreased with increasing applied potential. In addition, the ac impedance measurement was carried out to investigate the change of the amount of anion adsorption with Cl- ion. The variation of repassivation rate with chloride ion was discussed in terms of competitive adsorption reactions between OH- and Cl- ions. It is suggested that for the short exposure time the increase of current density by the addition of Cl- ions is caused by the inhibition of OH adsorption reaction due to the adsorption of Cl- ion on the nickel electrode. For the long exposure time, the decrease of the current density resulted from the fact that the adsorbed Cl- ion facilitates the place exchange reaction and thereby assists film formation.
In chapter VI, a model of the growth kinetics of passivating oxide film has been proposed, on the basis of the movement of metal cation vacancy under an electric field. The model included the charge transfer reactions at both metal/film and film/electrolyte interfaces. By neglecting the formation reactions of metal and oxygen vacancies at film/electrolyte interface, the 'point defect model' is obtained as special case in which the diffusion of oxygen vacancy was only contributed to the film growth. the theoretical equations derived from the model were confirmed by the results already reported by the Sato and Cohen for passive iron in borate solution. It is found that the field strength of the film increased from $2.24×10^5$ to $4.78×10^5Vcm^{-1}$ with increasing applied potential from 515 to 813 $mV_{SCE}$.
In chapter VII, the wear behaviour of PTFE composites containing various FEP contents rubbing against a AA-7075 aluminum alloy has been elucidated. Wear tests were carried out under constant sliding conditions: applied stress, 1.87 MPa; sliding speed, 1.3m $s^{-1}$. The wear resistance of PTFE composites were higher than that of pure PTFE within the range of all the experimental FEP contents(5-40 wt.%). Also, it is shown that the PTFE composite with minimum wear rate contains about 35 w% FEP. In order to investigate the weareducing mechanism, micro-hardness tests, differential scanning calorimetry (DSC) and micrography have been carried out. The Vickers hardness showed a little variation with FEP content. From the DSC results, the FEP peak was observed in the PTFE composite, but not in wear debris. Also, the molecular weight and crystallinity of PTFE composites containing various FEP contents did not change with FEP content, but those of the wear debris changed. The wear-reducing mechanism of PTFE composites containing various FEP contents is discussed in terms of load supporting action of FEP and the changes in the polymer film which is transferred onto metal counterface.
