ANOVA Study of Corrosion under Controlled Conditions of Heat Transfer

Lina Nael Saleem
Prof. Dr. Qasim J. Slaiman

The aim of present work is to use the analysis of variance (ANOVA) to study the effects of several variables on the corrosion process at different conditions of different metals under isothermal and heat transfer conditions. A study was carried out to investigate the effect of hydrodynamic variables as well as temperature and heat flux on corrosion of copper and iron metals.Two hydrodynamic geometries were investigated: cylinder in cross flow and rotating cylinder system under different ranges of temperature,Reynolds number and heat flux. The analysis of variance is used which involves comparison either of several different treatments or the influence of two or three factors at the same time.The data obtained from different electrochemical techniques were analyzed to determine the influence of Reynolds number, temperature and heat flux on cathodic region. The responses considered are limiting current density, mass transfer coefficient, heat transfer coefficient, and interfacial (skin) temperature. It was found that the limiting current density of carbon steel is influenced by flow more than temperature and therefore by a diffusion component of oxygen while the anodic limiting current density of copper is affected by temperature more than velocity under isothermal conditions. It is also shown that limiting current density of carbon steel and copper under heat transfer conditions is influenced by velocity followed by heat flux and then temperature to different extents at 0.01 and 0.05 significant levels.The mass transfer coefficient is basically flow dependent, because it increases as the rotation rate or Re increases under isothermal conditions. Its value  on copper in the anodic region is almost equally affected by hydrodynamics and temperature. It was found that mass transfer coefficients under heat transfer conditions are also basically influenced by velocity more than other variables.The heat transfer coefficients are influenced by bulk temperature followed by heat flux and then rpm or Re; it increases as temperature increases. The interfacial (skin) temperature is influenced basically by bulk temperature more than other variables, i.e., heat flux and velocity. These influences are dictated by ANOVA statistical analysis.Under isothermal conditions, the thickness of hydrodynamic boundary layer decreases as Re or (rpm) increases at constant temperature, also it decreases with increasing temperature at constant Re or (rpm). Diffusion boundary layer decreases as Re or (rpm) increases at constant temperature. On the other hand it increases with increasing temperature at a given Re or (rpm). The ratio between hydrodynamic and diffusion boundary layers is not affected by Re or (rpm), but it decreased with increasing temperature.Under heat transfer conditions, hydrodynamic and thermal boundary layer decrease as temperature increases at constant Re or (rpm). Also they decrease with increasing Re or (rpm) at constant temperature.Diffusion boundary layer increases with increasing temperature at constant Re or (rpm) and it decreases with Re or (rpm) increasing at constant temperature.The ratio of thermal boundary layer to diffusion boundary layer is higher than one, which shows that thermal boundary layer is always greater than diffusion boundary layer. As the diffusion boundary layer is appreciably smaller than the hydrodynamic boundary layer, this indicates a negligible convection within the diffusion layer. This means that in the major part of the hydrodynamic layer, motion of liquid completely levels concentration gradients and suppresses diffusion, but not perhaps within the thermal layer which may induce some convection in the diffusion layer and hence higher corrosion rates.