Phosphates, corrosion behavior

Corrosion of industrial alloys in alkaline waters is not as common or as severe as attack associated with acidic conditions. Caustic solutions produce little corrosion on steel, stainless steel, cast iron, nickel, and nickel alloys under most cooling water conditions. Ammonia produces wastage and cracking mainly on copper and copper alloys. Most other alloys are not attacked at cooling water temperatures. This is at least in part explained by inherent alloy corrosion behavior and the interaction of specific ions on the metal surface. Further, many dissolved minerals have normal pH solubility and thus deposit at faster rates when pH increases. Precipitated minerals such as phosphates, carbonates, and silicates, for example, tend to reduce corrosion on many alloys. 

The formation of hydrolysis products, in the case of zinc phosphate, depends on the permeability of the protective coating. The permeability of the protective coating itself is influenced by the type of resin used, and in particular, by the PVC (Pigment Volume Concentration). This means that the choice of resin, pigments and fillers and the complete formulation have an important influence on the corrosion protection behavior of protective coatings containing zinc phosphate [5.69]. 

Veleva, L., Chin, J., and del Amo, B., Corrosion electrochemical behavior of epoxy anticorrosive paints based on zinc molybdenum phosphate and zinc oxide, Progr. Org.Coat., 36, 211, 1999. 

Cyclic Voltammetry—5 V/min. Figures la-e present the j-U behavior for the first cycle of copper-2% zinc in phosphated saline and in protein solutions. The corrosion potentials (ia—ic) during the forward scans were between -0.35 to -0.40 V in all cases. Two anodic peaks were observed for all protein solutions at —0.25 to —0.10 V and +0.10 to +0.30 V. The first peak in the supporting electrolyte was also observed, whereas j continued to increase, never reaching a peak up to the reversal potential of +0.5 V. Two main cathodic peaks were observed at —0.30 to —0.45 V and —0.65 to —0.75 V in all cases. A prepeak inflection also occurred at -0.2 to -0.3 V for both the albumin and globulin systems, and a small peak at —1.1 to —1.2 V for most systems. Cathodic currents increase sharply below about —1.5 V. Figures 2a-b represent the surface appearances after the first cycle of polar-... 

It is obvious that in case of a defect down to steel, which leads to the enhanced anodic dissolution of zinc, the delamination of the purely alkaline cleaned galvanized steel surface is not faster than that of a phosphated surface. Such a behavior can be explained by an anodic delamination process. If the corrosion conditions are such that no formation of a cathode is possible in front of the anode, then just the kinetics of zinc dissolution determine the degradation of the polymer-metal composite. 

Both (PPy)s and (PPyXn films were prepared in the galvanostatic mode (j = 4 mA/cm ) and had thickness between 3 and 10 pm. The (PPyXn films heated at 180°C for 30 min in order to achieve polycondensation of the modified polymer chains. Both samples were over-coated with cath horetic paints (total thicloiess about 28 pm) and then compared in salt spray tests with phosphated steels covered with the same thickness of cataphoretic paint These tests were performed for 500 h, after which adherence was checked. It is found that the adherence properties are similar for all samples. The main difference is that corrosion propagation around the streaks is twice as small for (PPy)m than as for (PPy), ch shows behavior identical with tfiat of the phosphat steel system 02). 

Nowadays, the use of the reflection electron microscope (REM) or, recently, the tunnel electron microscope, as well as secondary ion mass spectrometry (SIMS), AES, electron-dispersive X-ray spectrometry, impedance spectroscopy, and so on, are yielding substantial increases in the knowledge of corrosion reactions in coatings and at their interface with metal or other substrates. As far as zinc or zinc-coated surfaces are concerned, problems of interfacial and intercoat adhesion, differential diffusion phenomena and electrolytic cell behavior on the substrate, and interreactions of zinc with conversion coatings (chromates, phosphates, silanes, silanols, etc.) have been analyzed, leading toward spectacular improvements in, for example, paint adhesion, absorption of conversion coatings and, in general, the protective action inside films as well as on their substrates. 

The behavior of fertilizers in relation to crop-spraying aircraft is examined by Marshall and Neubauer (1955), who rate copper or cobalt sulfides (either on their own or in superphosphates) as unsuitable for use with galvanized steel equipment. Ammonium phosphate (measured corrosion rate 250 p,m/year) and nitrolimes (corrosion rate 300 p,m/year) can be used only with care, for short times. Salt (100 p,m/year) is better, as is calcium phosphate, while chalk and bone meal have little effect on galvanized steel. 

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. 

Dissolved mineral salts. The principal ions found in water are calcium, magnesium, sodium, bicarbonate, sulfate, chloride, and nitrate. A few parts per million of iron or manganese may sometimes be present, and there may be traces of potassium salts, whose behavior is very similar to that of sodium salts. From the corrosion point of view, the small quantities of other acid radicals present, e.g., nitrite, phosphate, iodide, bromide, and fluoride, generally have little significance. Larger concentrations of some of these ions, notably nitrite and phosphate, may act as corrosion inhibitors, but the small quantities present in natural waters will usually have little effect. 

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