Influence of the processes of additional phosphate post-treatment of ceria conversion coatings deposited on Al 1050 on their corrosion protective behavior

Abstract

Systematic studies were carried out with respect to the elucidation of the role of additional “post-treatment” immersion phosphate processing on conversion corrosion protective ceria coatings CCOC deposited on aluminium Al 1050. Thereupon, the influence of various compositions of the CCOC as well as the conditions of such treatment were studied, based on solutions of Na3PO4 or NH4H2PO4. The results obtained with respect to the corrosion behavior, respectively the corresponding changes in the chemical composition of the studied corrosion-protective systems, give us the reason to suppose that the phosphate layers formed on the CCOC play the role of not only protective barrier coating, but they also change the kinetics of the conjugated electrochemical reactions characterizing the corrosion of aluminium. Key words: aluminum, pre-treatment, cerium-containing conversion films, corrosion

Phosphate sealing improvement of the corrosion resistance of thin cerium oxides coatings electroless deposited on aluminium

“…The similar post-treatment of the system Al(NaOH&HNO3)/CCOC(Ce+Cu) in both types of phosphate solutions leads to a substantial shift of the anodic curves towards a more positive Ecor and a lower icor; III. A characteristic feature of the anodic PDPC is the absence of a zone of passive state, which justifies the consideration that the corrosion potential (Еcor) of the studied systems coincides with the potential of pitting formation (Еpit), which is a prerequisite for the local character of the corrosion process under the conditions of stationary corrosion [2]. These results showed that the phosphated CCOC are not only static barrier coatings, but they change the kinetics of the conjugated electrochemical reactions, i.e., they also determine the electrochemical protection.…”

“…Our prior investigation [2] of the potentiodynamic polarization curves (PDPC) of the ongoing cathode and anodic processes in CM established that: I. The deposition of CCOC on Al(NaOH&HNO3) and the sealing of this system in NH4H2PO4 (formation of the mixed system Al(NaOH&HNO3)/CCOC(Ce+Cu)/PhL(NH4H2PO4)) determines the strong influence on the kinetics of the cathode reaction of reduction of oxygen, which enhances the protection degree of this system; II.…”

“…Layers of CCOC (prepared by chemical conversion treatment) were deposited on substrates of "technically pure" Al 1050. Their pre-treatment, as described in [2], involved degreasing in organic solvent, etching in 1.5 M NaOH and consecutive activation in 5 M HNO3 (the abbreviator of these samples in the text is "Al(NaOH&HNO3)"). The CCOCs were formed in a solution containing 0.5 M CeCl3×7H2O + 1×10 -5 M CuCl2×2H2O, as described in [3].…”

“…The post-treatment of the thin CCOC as-deposited on Al substrates aiming at the additional formation of phosphate layers (PhL(s)) on them was realized in 0.08 M Na3PO4 or 0.22 M NH4H2PO4, as described in [2]. The abbreviations in these cases are: "Al(NaOH&HNO3)/CCOC(Ce+Cu)/PhL(Na3PO4)" or "Al(NaOH&HNO3)/CCOC(Ce+Cu)/ PhL(NH4H2PO4)", respectively.…”

Growth process of molybdate conversion coating on the surface of aluminum foil and its adhesive mechanism

“…However, the use of hexavalent chromium is restricted by its high toxicity to humans and environmental contamination. 5 In order to overcome these problems, a number of alternative treatments are proposed using chrome-free solutions such as phosphate (P), 6,7 rare earth salt, 8 permanganate, 9 titanate/zirconate (Ti/Zr), 10,11 and molybdate (Mo). 12,13 However, some problems remain to be solved for these methods.…”