The temperature and anionic composition effect of close to neutral solutions on kinetics of aluminum local activation

摘要

The aluminum is widely used as a basis of construction materials in various technologic equipment contacting with aggressive media at elevated and reduced temperatures. The last demands accurate representations about its corrosion destruction mechanism in corresponding conditions complicated by presence on a surface of metal proof oxide-hydroxide film. This film has different composition and stability, depending on an external environment being exposed to local destruction. However, the data question, in spite of the actuality, hasn't received till now the univalent decision that has defined the purpose of the present work in which attempt of its detailed analysis taking account of role of anionic composition, pH and temperature of NaCI solutions of various concentrations is undertaken. Systems aluminum (Al 99. 9 %) - aqueous and borate buffer (0.2M H_3BO_3 + 0.05M Na_2B_4O_7) (BB) solutions with additives NaCI in the concentration range from 10-5 to 10-1 M and variation of pH from 4.8-5.6 to 8.4 were objects of research. The basic attention has been given kinetics of local activation (LA) metal at various temperatures (5 - 60o C). Complex of independent physic-chemical methods (electrochemical, microscopical and other) the same as [1] was used. The research of aluminum surface after electrochemical experiments at aqueous and BB solutions of NaCI has shown that under anodic polarization in these systems, metal is exposed to LA with the pit formation (PF) which depth depending on pH and temperature varies from 3 to 18 µm. So, in aqueous solutions NaCI the maximum depth of pit is 12 µm, and in BB electrolytes -18 µm. With growth of temperature from 5 to 60°C character of aluminum surface destruction remains, but diameter pits increases, and potential of LA (ELA) shifts to less noble of values coinciding with potential of corrosion (ECORR). As it is known [2], spontaneous transformation aluminum oxide-hydroxide film, representing amorphous aluminum hydroxide or crystalline bayerite, in less hydrated - boehmite - occurs at the critical temperature equal 60 - 70° C. It is possible to suppose, that this effect causes PF on aluminum at 60° C already under ECORR and LA passes in pitting corrosion. In this case, the temperature starts to act as the additional activator of local destruction process. This fact is confirmed by data of chronopotentiometric study under free corrosion condition, according to which aluminum activation begins through 8 min at 60 °C and through 4 min at 70 °C after immersion metal in aqueous solution NaCI without anodic polarization. The presence in a solution of borate-ions, apparently, can prevent the described above process of aluminum film transformation. In chosen for detailed research of the temperature factor role aqueous (pH 5.2) and BB (pH 8.4) solutions of NaCI at CNaCI = 10-3 M (where PF proceeds with average intensity) it has been established that rate of pit initiation (defined reverse value to the induction period of PF), exponentially increases with temperature growth, following Arrhenius-type plot.