The American Society for Testing and Materials (ASTM) is an independent organization devoted to the development of standards.

American Society for Testing and Materials

Treatment of rinse water from zinc phosphate coating by batch and continuous electrocoagulation processes

Alternative conversion coatings to chromate for the protection of magnesium alloys

Zinc phosphate coating was formed on AZ91D magnesium alloy through a phosphating bath where H3PO4, ZnO, Zn(NO3)2 and NaF were applied. Chlorate (NaClO3) was used as an accelerator of phosphatization to replace nitrite. The coating compositions were hopeite (Zn3 (PO4)2·4H2O) and Zn. The growth process of the phosphate film on the magnesium alloy substrate was investigated by SEM observation and XRD analysis. In the early stage (0–60 s) of phosphatization, both hopeite and metallic zinc nucleated and grew together on the substrate to form flower structure. A great number and uniform nucleation of hopeite and metallic zinc implied that they formed on the β and α phases of the AZ91D magnesium alloy simultaneously. Afterwards, because magnesium alloy substrate was almost fully covered, only hopeite deposited continuously to form slab-like structure. The formed zinc phosphate coatings exhibited high corrosion resistance in 5% NaCl solution as shown by polarization measurement.

/pdf/growth-of-zinc-phosphate-coatings-on-az91d-magnesium-alloy-4b8bqq938f.pdf

Growth of zinc phosphate coatings on AZ91D magnesium alloy

Localized corrosion of a 2219 aluminium alloy exposed to a 3.5% NaCl solution

Anticorrosive protection of Al alloys still depends heavily on the use of chromates, which are widely and universally employed as chromate conversion coating and chromic acid anodising pretreatments. The replacement of chromate based treatments with more environmentally compliant processes and materials has been identified as a high priority. The aim of this paper is to review the most recent developments in the application of common inorganic protection layers based on conversion coatings for Al alloys. The review lists and discusses the majority of conversion coatings, including those formed through anodisation, on Al alloys as potential replacements to the most widely used treatments based on chromate chemistry.

On conversion coating treatments to replace chromating for Al alloys: Recent developments and possible future directions

Zinc phosphate (ZPO) conversion coatings formed on 2024-T3 aluminum alloy, and on high-purity aluminum, after dipping in coating baths containing different amounts of Ni2+, have been studied by X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), energy-dispersive X-ray (EDX) spectroscopy, scanning Auger microscopy (SAM) and electrochemical measurements. Significant variations are observed in coating morphology and in stability against corrosion for the 2024-Al alloy as the amount of Ni2+ in the coating solution increases through the 0–2000 ppm range. By contrast, for high-purity Al, the most marked changes in coating morphology occur only as the Ni2+ concentration reaches 2000 ppm. The presence of additional elements, and especially Cu, in the 2024-Al samples results in an increase in the number of cathodic sites and hence more coating is formed for corresponding conditions. Additionally, the existence of second-phase particles in the 2024-Al sample leads to variations in the coatings at the microstructural level across the surface. These differences include local variations in the amount of Ni2+ in the coatings, as well as the local coverage.

A.S. Akhtar

Papers

On conversion coating treatments to replace chromating for Al alloys: Recent developments and possible future directions

The effect of Ni2+ on zinc phosphating of 2024-T3 Al alloy

The effect of pH and role of Ni 2+ in zinc phosphating of 2024-Al alloy

On the growth of conversion chromate coatings on 2024-Al alloy

Growth of permanganate conversion coating on 2024-Al alloy