Use of solutions of organic acids to produce low friction anodised surfaces

SUMMARY

The properties of anodic films produced by anodising aluminium in aqueous solutions of organic acids at room temperature at commercially interesting current densities were investigated with particular emphasis on the wear index, micro hardness, surface roughness and pore structure. Voltage time curves obtained using 10% w./v. solutions of the pure acids showed a rapid rise in voltage at commercially realistic current densities of 1–2 A dm⁻² but this problem could be overcome by the addition of 1–2% v./v. sulphuric acid. The films obtained under these conditions using sulphosuccinic acid (SSA), sulphosalicylic (SCA) or sulphophthalic acids (SPHA) had properties closely resembling those obtained under hard anodising conditions in 10% sulphuric acid at 0–4°C. Films were not formed using para-toluenesulphonic acid (PTSA) and the use of maleic acid (MA) resulted in the formation of substantial quantities of fumaric acid. The oxide films exhibited a structure made up of densely packed columns suffused by narrow pores. The films fall into two categories. On the one hand films produced using sulphur containing organic acids have very narrow pores of less than 10 nm diameter resembling those produced by the conventional low temperature sulphuric acid process, and on the other hand films produced using maleic or citric acids exhibit much larger pores with diameters of the order of 20 nm. Attempts to incorporate PTFE particles from aqeous dispersions of this polymer into the films were unsuccessful due to their large size (~200 nm). Duplex films with a highly porous outer layer (~100 nm diameter) on top of a dense hard layer were produced and evidence is presented which shows that some of the smaller PTFE particles can enter the mouths of these pores.