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Abstract

The subject of this doctoral dissertation is the corrosion protection of aluminium alloys. Aluminium alloys are important technological materials with excellent mechanical properties but prone to corrosion in chloride and other aggressive environments which may provoke the localized or uniform dissolution of the alloy surface. The susceptibility to corrosion originates from the microstructure of aluminium alloys which contain intermetallic particles added to improve aluminium’s mechanical properties; however, the presence of these particles stimulates the corrosion due to the difference in their electrochemical activity. Understanding the corrosion mechanism of aluminium alloys is important as it serves as a basis for postulating the subsequent methodology of their protection in aggressive environments. This dissertation was devoted to the (i) study of corrosion of aluminium alloys in a chloride environment with and without added sulphide as a pollutant, and the (ii) development and study of zirconium conversion coatings as a contemporary protection method for aluminium alloys. The methodology was versatile, including electrochemical measurements in 0.5 M NaCl solution and microstructural and surface analysis carried out using SEM-EDXS, TEM, XPS, ToF-SIMS, XRD and digital imaging technique.
Aluminium alloys containing silicon are suitable for the use in the marine environment. Under such conditions not only chloride ions are provoking corrosion; other pollutants may be aggressive as well. For example sulphide ions which are known to promote the corrosion of copper- and steel-based alloys. The behaviour of aluminium alloys in the presence of sulphide has not been investigated in detail. In this dissertation, the corrosion behaviour of AlSi7Mg0.3 cast alloy (A356.0) was studied in artificial seawater with and without added sodium sulphide. It simulated sulphide which can form as a product of decaying of a biological compound as a result of the action of sulphate-reducing bacteria. The addition of Na2S to artificial seawater was studied for concentrations from 20 to 100 ppm in the period of 40 days. Low sulphide concentration (<50 ppm) in seawater improved the corrosion resistance of the substrate which was ascribed to the slower kinetics of oxide formation leading to a more dense, crystalline and compact aluminium-based oxide layer. At higher concentrations, the surface layers formed were thicker and contained an increased content of magnesium hydroxide. These layers initially provided increased protection but became porous with immersion time. Therefore, in terms of corrosion protection, most compact layers were formed in the presence of lower sulphide concentrations in seawater.
In order to outperform traditional corrosion protection based on chromate conversion coatings, contemporary methods have to be as efficient but environmentally friendly. An example of the contemporary coatings are zirconium conversion coatings (ZrCCs). Their formation was investigated as a parameter of a conversion bath: concentration of hexafluoric acid and conversion time. Appropriate conversion conditions were determined for different aluminium alloys. It was reported herein for the first time that ZrCCs applied on aluminium alloys show a self-sealing/active corrosion effect during immersion in 0.5 M NaCl resulting in improved corrosion resistance with time. This effect was ascribed to the leaching of fluorine out from the ZrCC, which leads to the transformation of ZrF4 / ZrOxFy into ZrO2∙xH2O and the formation of aluminium oxide/hydroxide; the latter also reacts with major elements from the substrate, i.e. Mn and Si. These changes resulted in the closing of nanopores in ZrCC and reinforced the coating/substrate interface against dissolution. The self-sealing effect depends on conversion time and the type of aluminium alloy substrate, i.e. the major alloying elements in the alloy. At appropriate conversion time, the self-sealing effect was most pronounced for AA3005 (Mn-containing alloy) followed by AA356.0 (Si-containing alloy) and AA5754 (Mg-containing alloy). In contrast, corrosion resistance of ZrCC applied on AA2024 (Cu-containing alloy), and AA7075 (Zn-containing alloy) decreased during immersion in 0.5 M NaCl solution. Therefore, self-sealing zirconium conversion coatings provide excellent corrosion protection of aluminium alloys containing manganese and good protection for those containing silicon and magnesium. The same coatings provide only poor protection of aluminium alloys containing copper and zinc.