Anodic Film Growth by Anion Deposition in Aluminate, Tungstate, and Phosphate Solutions

Abstract

Films were obtained by anodic treatment of A1, Mg, Ni, Fe, Zn, Bi, Cd, Co, and Cu in 0.1N NaA102 and A1, Bi, Cd, Cu, and Zn in 0.1N Na~WO4. A formation voltage of 30v was applied for 10 rain in all cases except A1 in NaA102 where the maximum formation voltage was 100v. The anodic films were studied by electron diffraction and chemical analyses. Films obtained in NaA102 solution appeared to be ~-A120~. 3H20, and those obtained in Na2WO4 solution were oxides of the anode metal, or mixtures of the anode metal oxide with WO3. The incorporation of phosphorus compounds in anodic films formed on A1 in 0.1N Na2HPO4 solutions was also observed. The mechanism of anion dep-osition and film growth in these solutions is discussed. In a previous paper (1) there were described a number of anodic spark reactions and products in solutions of NaA10~, Na2WO4, and Na2SiO3. The occurrence of these reactions showed that a high-voltage barrier film is present on the anode, but the composition and properties of the barrier below the spark potential were not investigated. This paper is concerned with the composition, thickness, and structure of the anodic films which form below the spark potential on a variety of metals in solutions of NaA102 and Na2WO4. Formation of anodic films on A1 in Na2HPO4 solution was also investigated. Experimental The materials and apparatus employed in this investigation were the same as those in the previous paper (1), with the addition of reagent grade Na2HPO4 and 99% pure H~PO~ which had been prepared at the Oak Ridge National Laboratory and contained a high level of radioactive phosphorus-32. Anode geometry for the study of anodic film growth on Ni in NaA102 solution and for the study of HPO4 = incorporation in anodic films on A1 was also the same as in the previous paper. Anodes on which films were grown for electron diffraction study were made with the end to be anodized machined to form a flat plate which could be detached and mounted in the electron diffraction sample holder. Anode cleaning procedures were in all cases as described previously. Electron diffraction patterns were obtained for films grown on various anodes in 0.1N NaA102 and 0.1N Na2WO4. The anodic coatings were formed at a maximum current density of 0.016 amp/cm 2 up to a formation voltage of 30v, with the exception of the coating on the aluminum anode which was formed at 100v. The coated anodes were washed in distilled water, dried, and then inserted in the electron dif-fraction sample holder. An RCA 100 kv Model EMU 3D electron diffraction unit was employed for these exposures. 1 The diffraction patterns were measured 1 The electron diffraction patterns were prepared at the Aeronautical Materials Laboratory, Naval Air Materials Center, Philadelphia, Pennsylvania. 853 on a Model-A circular film measuring instrument made by the Chas. Supper Company. Information as to the film thickness and quantity of the anion constituent deposited in the water insoluble anodic films was obtained by two methods employing (a) nickel anodes in NaA1Oe solution, and (b) aluminum alloy anodes in radioactive Na2HPO4 solution. Nickel anodes having apparent surface areas of either 7.9 or 2.85 cm e were anodized in 0.1N NaA102 for periods of 10 min with final formation voltages of 10, 30, andl00v. With each anode the voltage was raised from zero to the final value by manual adjustment of the power supply at a rate sufficiently slow to maintain the current density below 0.04 amp/cm 2. After anodizing, the anodes were washed in distilled water and then soaked in 1-1 HC1 to dissolve the anodic coating. Resulting solutions contained aluminum chloride together with small amounts of nickel chloride. The aluminum content was determined gravimetrically following precipitation with 8-hydroxy-quinoline. The nickel in the sample was complexed with tartaric acid to prevent its precipitation. Similar analyses were conducted with blanks prepared from nickel rods which were not anodized but which were soaked in distilled water, 1-1 HC1, or 0.1N NaA102. In order to calculate the thickness of the aluminum containing portion of the anodic film, it was assumed that the aluminum compounds had a density of 2.5 g/cm 3 which is the value for fl-A1203-3H20. The amount of phosphate incorporated in anodic films on 7075 alloy aluminum anodes was measured using an 0.1N NaH2PO4 solution which contained radioactive P-32. This experiment was similar to one reported by Plumb (2). The anodes for measurement of phosphate incorporation in anodic films consisted of round 11 mm 7075 alloy aluminum rods. Disks, having one ano-dized flat surface, were cut from these rods, and counts were made on the disks with a Tracerlab Model SC-18A scaler counter which monitored the) unless CC License in place (see abstract). ecsdl.org/site/terms_use address. Redistribution subject to ECS terms of use (see 132.174.255.116 Downloaded on 2015-03-08 to IP