Microelectrochemical Studies of Pit Initiation on High Purity and Ultra High Purity Aluminum

Abstract

The onset of pitting on high purity and ultra high purity aluminum has been studied in the micrometer range by using a microelectrochemical cell in order to evaluate influences that induce localized corrosion. Microelectrochemical tests on polished samples of Al 99.999 and Al 99.9999 in 1 M NaCl showed that pitting at large areas occurred at several hundreds mV more negative than at small areas, suggesting that even very pure Al contains weak points. On rough surfaces weak points are more “activated” than on smooth surfaces. Locally measured pitting potentials of polished samples were shifted to more positive values than those measured on ground samples. Larger areas contain always small scratches, even on polished samples. Hence, the surface roughness showed a minor influence on the onset of pitting on larger areas. Microelectrochemical measurements performed on pure Al 99.99 with small amounts of copper showed that a copper content below 30 ppm is too small to have a beneficial or detrimental effect, whereas a copper content above 300 ppm probably leads to the formation of small inclusions or precipitations that acted as preferential corrosion initiation sites. A copper content around 100 ppm showed a beneficial effect on very small areas but not on larger ones.

Simultaneous effect of surface roughness and passivity on corrosion resistance of metals

“…The same effect was also observed for the effect of roughness on corrosion rate for AE44 Mg alloy before [7]. In both cases the metal has no ability to form a passive layer but in the case of other metals which form a passive layer, a reverse trend was observed [1][2][3][4][5][6]. As it is shown in Figure 1, polarization curves rise to parallel and it is clear that both cathodic and anodic branches show a lower current density indicating that the hydrogen evolution reaction is activation controlled [10,11].…”

“…In Figure 3(e)-(h), a reverse trend is observed for the nickel samples in 0.5 M H2SO4 solution namely more degradation on rougher samples and corrosion seemed to be concentrated along the grooves [12]. This trend in change of corrosion versus surface roughness for nickel as an active-passive metal was observed by researchers in the case of stainless steel and aluminium [3,5] which showed poorer corrosion resistance and trapping the corrosive ions on rougher surfaces of active-passive metals.…”

“…The results are in good agreement with SEM and potentiodynamic polarization tests. In the case of metals such as aluminum and nickel, the lower corrosion rate was related to the formation of a stable passive film on smoother surfaces as suggested by the authors [3,[12][13][14][15] but in the case of mild steel no stable passive film is formed and rougher surfaces showed less corrosion and lower oxygen content. Generally, more increase in the oxygen content was observed for mild steel indicating more oxides forms on the surface.…”

“…Such a trend was observed for copper, nickel, aluminium, stainless steel, magnesium and titanium alloys [1][2][3][4][5][6]. In all cases the effect of creating different roughnesses were investigated on both localized and general corrosion performance of the alloys.…”

A Summary of Corrosion Properties of Al-Rich Solid Solution and Secondary Phase Particles in Al Alloys

“…Furthermore, these basic corrosion data can be used to simulate pit initiation and propagation. One very successful method for determining the corrosion characteristics of secondary phase particles is the electrochemical microcell method, which uses a small capillary filled with electrolyte, the tip of which permits electrochemical analysis of specific particles [11,[35][36][37][38][39][40]. This method has been employed with both synthesized intermetallic compounds (IMCs) and/or actual alloy substrates [11,17,34,[41][42][43][44][45][46][47].…”