Zinc Phosphating-Spray

First comes the pretreatment stage. After rust removal and alkaline degreasing, a zinc phosphate formulated pretreatment (see Section 15.2) is applied by dip or spray-dip. Crystalline iron-rich zinc phosphate forms on the metal surface at a coating weight of 0.5-4.5 g/m. ... 

Good quality steel is used and electrozinc is preferred for washing machines. Steel is pretreated with iron phosphate for economy electrozinc with a fine crystal zinc phosphate. No primer is normally used 25-40/im of finish is applied direct to metal. The required properties are best obtained with a thermosetting acrylic or polyester/melamine-formaldehyde finish. Self-reactive acrylics are usually preferred these resins contain about 15 Vo 7V-butoxymethyl acrylamide (CH2=CH —CO —NH —CHj—O —C4H,) monomer and cure in a manner similar to butylated melamine-formaldehyde resins. Resistance or anti-corrosive properties may be upgraded by the inclusion of small amounts of epoxy resin. Application is usually by electrostatic spray application from disc or bell. Shapes are complex enough to require convected hot-air curing. Schedules of 20 min at 150-175°C are... 

Zinc phosphate primer in quick- Airless spray. 2 80-100... 

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. 

Similarly in Japan there has been a strong move towards full dip treatment and over 50% of car body lines now employ this method. In Europe, while there are some dip-only plants, the majority of recent installations have presprays prior to the dip tank. In the USA spray-only plants still predominate. Zinc phosphate processes normally operate in the range 50-60°C. Low temperature processes operating at 25-35°C are widely used in the UK and Italy but have not been extensively adopted elsewhere. 

Our electrochemical work differed drastically from the Groseclose work in that polymer coated metal samples were employed. Furthermore, we found that coatings can have corrosion resistance when their rest potentials are either more noble (B210/NVP) or less noble (B210/GBL) than the uncoated substrate. Leidheiser (22,23) examin zinc phosphate pretreated panels with automotive primer after 10 days exposure to the salt spray. The start and finish rest potentials of the samples with good paint performance were consistently more negative than those samples with poor paint performance ... 

On zinc phosphate conversion coatings, the "Mannich" derivatives of poly-4-vinylphenol have demonstrated performance equivalent to chromic chromate systems in salt spray, humidity, and physical testing. In addition. Table III illustrates results observed with automotive body paint systems evaluated by the "scab" or "cycle" test which causes failure more typical of actual end use conditions than do salt spray evaluations. Again, results equivalent to chromic-chromate post-treatments were obtained. In addition, the humidity resistance and adhesion tests were essentially equivalent to the chromium controls. 

Salt-spray data (for nos. 1 and 2 in Table V) indicated that, on zinc-phosphated steel, cathodic ED indeed led to considerably better corrosion resistance. On bare steel, however, both cathodic and anodic coatings failed completely. Further, we prepared a number of different, amine-modified epoxy resin esters containing between 30 and 50 %w of drying fatty acids (e.g. no. 3 in Table V). These could all be deposited cathodically and again attained excellent salt-spray ratings on phosphated steel but performed poorly on bare steel. We concluded that cathodic ED prevents phosphate layer degradation (a well-known phenomenon with anodic ED) and thus leads to superior corrosion resistance on pretreated steel. On bare steel the binders performed too poorly to allow comparison between cathodic and anodic ED. 

Donaldson [1988] also reports two examples where magnetic devices were installed in zinc phosphate spray systems where evidence was found for ... 

Isopropanolamine 2-Mercaptoethanol 1-Methyl-3-phenyl-5-[3-(trifluoromethyl) phenyi]-4(1 H)-pyridinone a-Naphthylamine Nitrosophenylhydroxylamine ammonium salt Phenyl benzoate m-Phenylenedimaleimide N-Phenylmaleimide Phenyl mercuric bromide Potassium carbonate Potassium N-hydroxymethyl-N-methyldithiocarbamate Pyridine Silica, fused Tetrachlorophthalic acid Thiourea p-Toluic acid , N-Tridecyl-2,6-dimethylmorpholine Trimethyl phosphate agric. chemicals, maleic hydrazide Hydrazine agric. sprays Zinc sulfate monohydrate agric., biodegradable Poly (hydroxybutyrate-hydroxyvalerate) agriculture... 

Table 10.2 presents as an example the basic characteristics of zinc phosphate dried in a pilot spray dryer with reference to industrial standards (Lyulin, 1998). It is evident that pigments dried in a spray dryer with the shock-wave atomizer satisfy all requirements for the commercial product. In addition, pigments obtained with the shock-wave atomizer have a narrower size distribution so the throughput of the ball mills used in a downstream process of standardizing is increased by about 20% to 30%. 

Zinc phosphate coatings are crystalline and somewhat thicker (1-5 gm ). The bath or spray contains primary zinc phosphate, Zn(H2P04)2, phosphoric acid and oxidizing accelerators. The pretreatments operate between pH 1.5 and 3.3 at temperatures between 25 and 90°C. The process begins with acid attack on the steel, oxidation (assisted by the accelerator) and precipitation of some ferric phosphate as above. However, the zinc phosphate is in a finely balanced equilibrium with the other species ... 

Grit-blast to Sa2 /2, apply drying oil paint system two coats zinc phosphate primer applied by brush followed by two coats incorporating MIO pigments by airless spraying (total nominal thickness 165-190 p,m) SF7... 

Lyon, Thompson, and Johnson [56] point out that the high sodium chloride content of the salt spray test can resnlt in corrosion morphologies and behaviors that are not representative of natnral conditions. Harrison has pointed out that the test is inappropriate for use on zinc—galvanized snbstrates or primers with zinc phosphate pigments, for example — becanse, in the constant wetness of the salt spray test, zinc undergoes a corrosion mechanism that it wonld not nndergo in real service [57]. This is a well-known and well-docnmented phenomenon and is discnssed in depth in chapter 7. 

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