Zinc phosphate effect

Some pigments exert an inhibitive effect on the corrosion of metal. The mechanisms are complex and not always fully understood. Inhibitive pigments include red lead, zinc phosphate and zinc chromate. 

The main value of salt-spray tests is in the evaluation of the effectiveness of phosphate coatings in restricting the spread of rust from scratches or other points of damage in a paint film. This feature is of particular interest to the motorcar industry, as vehicles are often exposed to marine atmospheres and to moisture and salt when the latter is used to disperse ice and frost from road surfaces. Great care is needed in the interpretation of a salt-spray test, as it has been found to favour thin iron phosphate coatings more than is justified by experience with natural weathering. In the motorcar industry the present custom is to use zinc phosphate coatings on the car bodies and all other parts exposed to the outside atmosphere. 

In the hydraulic transport of solids through steel pipelines, inhibitors of the sodium-zinc-phosphate glass type have been shown" to be effective. In the case of coal slurries the polyphosphate type was rejected because the de-oxygenating action of the coal lowered the inhibitor effectiveness. Hexavalent chromium compounds at 20 p.p.m. were more effective". ... 

Attempts have been made to improve the mechanical properties of these cements by adding reinforcing fillers (Lawrence Smith, 1973 Brown Combe, 1973 Barton et al, 1975). Lawrence Smith (1973) examined alumina, stainless steel fibre, zinc silicate and zinc phosphate. The most effective filler was found to be alumina powder. When added to zinc oxide powder in a 3 2 ratio, compressive strength was increased by 80 % and tensile strength by 100 % (cements were mixed at a powder/liquid ratio of 2 1). Because of the dilution of the zinc oxide, setting time (at 37 °C) was increased by about 100%. As far as is known, this invention has not been exploited commercially. 

The actions of zinc and aluminium differ. In general, metal ions such as zinc merely serve to neutralize the acid and are present in solution as simple ions (Holroyd Salmon, 1956 O Neill et al., 1982). But aluminium has a special effect in contrast to zinc, it prevents the formation of crystallites during the cement-forming reaction in zinc phosphate cements. 

Figure 6.3 The effect of environmental conditions on the surface of a zinc phosphate cement (d) stable and undulating surface with no sign of crystallites observed under dry conditions, (b) crystal growth observed in an atmosphere of 100 % relative humidity, (c) extreme porosity observed in the bulk of the cement pores are 0-5 pm in diameter (Servais Cartz, 1971).

Figure 6.4 Effect of pH on the elution of phosphate from a zinc phosphate cement mixed at two different consistencies (Wilson, Kent Lewis, 1970).

Figure 6.5 Effect of water content of the liquid (Hj0 HjP04) on the properties of a zinc phosphate cement (Womer Docking, 1958).

It is superior to the zinc phosphate cement for bonding orthodontic bands to teeth (Clark, Phillips Norman, 1977). It has greater durability and there is less decalcification in adjacent tooth enamel. This latter beneficial effect must arise from the release of fluoride which is absorbed by the enamel, so protecting it in a clinical situation where caries-produdng debris and plaque accumulate. 

Williams, J. I., Gates, G. L., Hembree, J. H. MacKnight, J. P. (1979). The frozen-aluminium-slab mixing technique its effect on zinc phosphate cements. Jourrml of Dentistry for Children, 46, 398-403. 

Windeler, A. S. (1979). Powder enrichment effects on film thickness of zinc phosphate. Journal of Prosthetic Dentistry, 42, 299-303. 

With regards to the conversion coated substrate, Wittel (25) observed that at temperatures greater than 140°C, tetrahydrate zinc phosphates lose part of their water of hydration. It is likely that the water of hydration liberated in the phosphate recrystallization process has a negative effect on the adhesion of the polymer matrix to the B40 panel. 

Since the choice of surface pretreatment prescribed for a metallic adherend has a direct effect on the performance of a joint in humid conditions, four types of commonly utilized automotive surface preparations were examined. The effects upon durability of no cleaning, alkaline cleaning, lubricating or zinc phosphating were examined. Accordingly, adherends were prepared using one of the four methods detailed below. 

In the final step of protection, the largest diffusion resistance has been found for the paint modified with zinc phosphate. It reflects the highest difficulties In the mass transport throughout this coating. Also observed Is a positive effect of zinc phosphate on the foirmatlon of sealing and Inhibiting zinc compounds... 

The flame retardant mechanism of PC/ABS compositions using bisphenol A bis(diphenyl phosphate) (BDP) and zinc borate have been investigated (54). BDP affects the decomposition of PC/ABS and acts as a flame retardant in both the gas and the condensed phase. The pyrolysis was studied by thermogravimetry coupled with fourier transform infrared spectroscopy (FUR) and nuclear magnetic-resonance spectroscopy. Zinc borate effects an additional hydrolysis of the PC and contributes to a borate network on the residue. 

The mechanism of the action of zinc phosphate is shown in Figure 69. Zinc phosphate dihydrate pigment is hydrated to the tetrahydrate in an alkyd resin binder [5.84], The tetrahydrate is then hydrolyzed to form zinc hydroxide and secondary phosphate ions which form a protective film of basic iron(III) phosphate on the iron surface [5.80]. The anticorrosive action of zinc phosphate depends on its particle size distribution. Micronization improves the anticorrosive properties [5.85]-[5.87], The effect of corrosion-promoting ions on the anticorrosive properties of zinc phosphate is described in [5.88], [5.89],... 

The aluminum in the zinc phosphate cements was considered very important, van Dalen [21] recognized its importance first. The reaction of zinc oxide and phosphoric acid was greatly moderated by aluminum. This effect was attributed to formation of an aluminum phosphate gelatinous coating on zinc oxide particles. In fact, Wilson and Nicholson believe that the gelatinous substance may even be zinc aluminophosphate phase [3], which subsequently crystallizes into hopeite and aluminophosphate amorphous gel (AlP04-nH20). 

In parallel to the work on zinc phosphate cements, porcelain dental cements also were developed. Steenbock [23] was the first to produce silicophosphate dental cement using 50 wt% concentrated phosphoric acid solution and an aluminosilicate glass. Schoenbeck [24] introduced fluoride fluxes in these glasses and vastly improved the dental cements. Fluorides lower the temperature of fusion of the glasses used in forming these cements. The same fluorides impart better translucency to the cement, and have some therapeutic effects. As a result, fluorides have become a part of modern dental cements. 

The performance properties of zinc phosphate pigments are attributed to chemical effectiveness, and the ability to form adhesion and inhibitor complexes on the surface of the substrate. In addition, in the case of zinc phosphate, electrochemical effectiveness, preferably in anodic areas, is also of note, since small amounts of the zinc phosphate will hydrolyze under moist conditions. The result of this reaction is argued to be the formation of zinc hydroxide and secondary phosphate ions, which are able to build protective layers on the metal surface in anodic areas [5.67, 5.68]. 

It is also discussed in the literature that, under moist conditions, basic complexes can be formed by reaction of zinc phosphate with inorganic ions or with carboxylic groups of the resin used, which lead, by reaction with metal ions, to so-called adhesion, cross-linking and inhibitor complexes [5.67-5.69]. Under the assumption, that the hydrolyzation process is the prerequisite for the effectiveness of zinc phosphates, this means that such pigments need a certain time before becoming active [5.69]. 

This leads to the conclusion that zinc phosphates do not have the well-recognized electrochemical effectiveness of chromate pigments [5.69]. 

Maybe, the proposed mechanism of the action of zinc phosphate is more related to theoretical considerations than to fully proven knowledge. However, practical experience has shown that zinc phosphate is an active anticorrosive pigment, but the protective effect of lead and chromate pigments can only be achieved in certain systems [5.70]. 

By controlled chemical modifications under consideration of different points of view with suitable elements and compounds connected with the optimization of the manufacturing processes, it has become possible to improve the effectiveness of zinc phosphate for many applications [5.68, 5.70]. 

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