Phosphate Conversion Coatings Research Articles

Phosphate Conversion Coatings - A Metal Pretreatment Process

Phosphate Conversion Coatings - A Metal Pretreatment Process

An electrochemical investigation of the development of phosphate conversion coatings

Potential and corrosion rate monitoring have been used to follow the growth of phosphate conversion coatings on mild steel, with impedance spectroscopy being utilized to examine the complete coatings. Significant residual metal corrosion rates are observed after coating completion. Impedance spectroscopy indicates similar surface processes after coating completion in both normal and accelerated baths. Analysis of the data indicates that only one electrode process is contributing to the low frequency response of the system, rather than the more general case of two, anodic and cathodic, parallel processes.

Advanced poly(acrylic)acid-modified zinc phosphate conversion coatings: use of cobalt and nickel cations

The incorporation of cobalt (Co2+) and nickel (Ni2+) ions in a poly(acrylic)acid (p(AA))-modified zinc phosphate solution promoted the growth and development of crystalline zinc phosphate (Zn·Ph) conversion coatings deposited on steel surfaces. This enhancement was due primarily to the preferential uptake of Co2+ and Ni2+ ions by p(AA), to form the Co- and Ni-OOC- salt complexes, rather than complexes with zinc ions. The electron trapping behaviors of Co2+ and Ni2+ dissociated from the salt complexes, and the hydroxides in the NaCl solution inhibited the cathodic reaction on Zn·Ph. In particular, the conversion coatings derived from the nickel-modified phosphate solution were characterized by the formation of a coating consisting of hopeite as a major crystal phase, zinc orthophosphate dihydrate as a minor phase, and an amorphous iron-rich phosphate phase; this coating provided an extensive coverage over the steel, which contributed to protecting it from corrosion.

CHARACTERISTICS OF TRANSITION METAL-ADSORBED ANHYDROUS ZINC PHOSPHATE COATINGS AS CORROSION BARRIERS FOR STEELS

Abstract In attempting to develop anhydrous zinc phosphate (Zn-Ph) conversion coatings suitable for use with a high-temperature performance polyphenylane sulfide (PPS) topcoat, the characteristics of Znfh deposited by immersing the steel in transition Co, Ni, and Mn cation-incorporated phosphating solutions were investigated. Two important features for the anhydrous 340°C-heated ZnPh were addressed. One was to determine if the electron trapping behavior of Co2+ and Ni2+ ions adsorbed in the crystal lattices acts to inhibit the cathodic reaction on the Zn-Ph, and the second was to determine! the less susceptibility of the a-Zn3(P02)2 phase to alkali-induced dissolution. The former affects the lifetime of the Zn-Ph layer as a barrier to corrosion of steel, and the latter reflects on the reduction of cathodic delamination rate of the PPS film from the a phase-steel substrate.

A novel, two-phase medium for electrochemically induced conversion coatings

A method to facilitate electrochemical coatings from a non-aqueous, poorly conductive (and therefore normally difficult to work with) electrolyte is presented. The process involves loading the electrolyte with pretreated ion-exchange beads to provide multiple low-resistivity paths for ionic but not electronic conduction between a counter-electrode and workpiece. Using the formation of iron phosphate conversion coatings from an oil-based lubricant as a test case, the effects of bead moisture content and oil additive concentration on the coating process are described. With this new approach electrochemical coating is possible, even in poorly conductive fluids, over a far larger range of electrode separations than could otherwise be accomplished.

The mechanism of chemical activation with titanium phosphate colloids in the formation of zinc phosphate conversion coatings

The activation of steel with colloidal titanium phosphate in the zinc phosphating process was studied. The mechanism of the activation has been clarified.The colloidal solution contains disc-shaped particles of the chemical composition Na4TiO(PO)2·O–7H2O. These particles are adsorbed on the metal surface during the activation period. When exposed to the zinc phosphating solution, sodium ions on the surface of the titanium phosphate particles are ion exchanged with zinc ions. The ion-exchanged particles, having a surface consisting of zinc phosphate, act as nucleation agents during the zinc phosphating.The disc-shaped particles are predominantly adsorbed with the flat surface against the metal surface. This causes orientation of the zinc phosphate crystals which are nucleated on the titanium phosphate particles. The adsorbed titanium phosphate particles can also affect the composition of the zinc phosphate layer.

アルミニウムのりん酸クロメート処理皮膜性状に及ぼす加熱の影響

Influence of heat treatment on organic coating adhesion and corrosion resistance of chromate phosphate conversion coatings was studied. Organic coating adhesion was evaluated on the basis of peeling strength of epoxy phenol paints at peel angle 180°. Corrosion resistance was investigated by means of salt spray test and retort test. Heat treatment at 200°C lowers adhesion property and corrosion resistance of the coatings. Studies by XPS, IR and DTA before and after heat treatment show that the main composition of the coatings is CrPO42-5H2O and that bound water of CrPO42-5H2O is dehydrated by heat treatment at 200°C×10min. TEM observation of the cross section of the coatings shows microcrack formation by heat treatment. Deterioration of adhesion property and corrosion resistance of the coatings by heat treatment is estimated to result from dehydration and microcrack formation of the film of conversion coating.

Aspects of the adhesion and corrosion resistance of polyelectrolyte-chemisorbed zinc phosphate conversion coatings

The ability of polyelectrolyte macromolecules to suppress the crystal growth of zinc phosphate (Zn · Ph) conversion coatings depends primarily on the functional pendant groups. The extent of segmental chemisorption of macromolecules having carboxylic and sulphonic acid groups on the embryonic crystal faces was found to be considerably higher than that of macromolecules containing amine groups. The reaction products formed by intermolecular reactions between amide groups in polyurethane coatings and carboxylic acid groups on the outermost surface of polyelectrolyte-modified Zn · Ph in Zn · Ph-to-polymer adhesive joint systems played an essential role in developing interfacial adhesive forces. A highly dense precipitation of Zn · Ph derived from a zinc orthophosphate dihydrate-based phosphating solution contributed significantly to reducing the corrosion rate of cold-rolled steel. It also was determined that the presence of an internally diffused polyelectrolyte in the Zn · Ph layers further enhances the resistance to corrosion of Zn · Ph itself.

Effect of Alkali Metal Hydroxides on the Dissolution Behavior of a Zinc Phosphate Conversion Coating on Steel and Pertinence to Cathodic Delamination

Abstract Raman microprobe spectroscopy, scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), and wet chemical methods of analysis were successfully used to determine the effects of high-pH environments on zinc phosphate conversion coatings. The more rapid dissolution of phosphate ions from the phosphate coating was pH dependent. At a pH of 12.5, phosphate ions were more rapidly dissolved from the phosphate coating while at 11.5 and 13.5 pH, zinc ions were more rapidly dissolved from the coating. Raman spectra confirmed the more rapid dissolution of the phosphate ions was a result of zinc oxide formation on the surface of the original phosphate conversion coating. The cation of the alkaline environment also proved to be an important factor in the dissolution of the phosphate conversion coating. Sodium hydroxide dissolved the phosphate coating at the greatest rate followed by hydroxides of potassium, cesium, and lithium. This cation dependence was related to the number of precipitation ...

Corrosion and safety aspects of steel diving cylinders

Aqueous internal corrosion of steel Scuba tanks is critically considered as a cause of explosive accidents during charging, storage and use of air cylinders. More restrictive safety regulations are shown to be needed in relation to the extremely high corrosion rates induced by high oxygen pressures. Accidental sea water entries are particularly dangerous because of tubular type localized attacks occurring in the presence of chlorides. Consideration is given to possible protective measures by phosphate conversion treatments and coatings.