Pitting potentials temperature effect

Bond, A. P., Effects of Molybdenum on the Pitting Potentials of 18<7oCr Ferritic Stainless Steels at Various Temperatures , J. Electrochem. Soc., U8, 208c (1971)... 

The aggressivity of halides varies, with bromide and chloride being most aggressive. Increasing concentration of the halide also depresses the pitting potential as demonstrated for two steels in Fig. 3.16. Certain ions in solution act as inhibitors (e.g. nitrate) raising the pitting potential while others depress it (e.g. sulphide). Temperature and pH also have effects as illustrated... 

Fig. 14. Effect of the temperature on the pitting potential of Alloy 600 in lO ppm Cl solution of pH 5 and 9 [34]. Reproduced with permission.

The potential importance of the temperature effect on surfactant properties has been recognized for some time and led to the concept of using the PIT as a quantitative tool for the evaluation of surfactants in emulsion systems. As a general procedure, emulsions of oil, aqueous phase, and approximately 5% surfactant were prepared by shaking at various temperatures. The temperature at which the emulsion was found to be inverted from o/w to w/o (or vice versa) was then defined as the PIT of the system. Since the effect of temperature on the solubility of nonionic surfactants is reasonably well understood, the physical principles underlying the PIT phenomenon follow directly. 

In electrochemical experiments for the determination of the pitting potential one either controls the potential (potentiodynamic method) or, more rarely, the current (galvanostatic method). The composition and temperature of the electrolyte are selected such as to represent the real environment to which the metal will be exposed, but without the oxidant present, whose effect is simulated by the anodic polarization. 

Figure 7.57 Effect of temperature on pitting potential of stainless steels in 3% NaCl [30],...

Studies in room-temperature iodide solutions have revealed anodic pitting potentials of +1.7 to +1.8 V, with little effect of acidification indicated (Ref 1,4). Above 40 to 50 °C (100 to 120 °F), values near +0.5 V (versus SCE) are reported. 

In summary, the existing work suggests that fluid has little effect upon the critical potential for passivity breakdown on aluminium, iron. Type 304SS, and Type 304LSS at ambient temperature, as well as on solution-annealed Type 304SS, sensitized Type 304SS, and mill-annealed Alloy 600 in chloride-containing solutions at elevated temperatures. However, fluid flow may affect the location of pit nucleation (in some... 

The effect of Cr is beneficial not only for stainless steels but also for Ni base alloys like Alloy 22 and Hastelloy C4 with ca. 20% Cr. These alloys are resistant to general corrosion and pitting, even in highly chloride+ontaining solutions like Q brine (>8 M Cl ). Pitting is obtained at elevated temperatures at sufficiently positive potentials only [40]. This is again a consequence of the presence of Cr, which... 

Rain or high relative humidity (which can cause condensation) increases corrosion of zinc that is roughly proportional to the time of wetness in a particular environment. Factors that determine time of wetness have been discussed by Grossman (1978). Temperature generally has only an indirect effect in air the relative humidity usually drops as temperature increases. At humidities exceeding 75%, corrosion rates rise perceptibly. A drop in temperature has the opposite effect. In solutions, reaction rates increase with temperature but—more important—in hot tap water there is a potential reversal between zinc and steel typically between 60 and 90°C, which then limits the benefits of a zinc coating to its barrier effect. In addition, localized pitting may occur, as noted in Section IV (How Zinc Can Protect Steel). 

---