Corrosion resistance, pretreatments

Many studies have shown that surface pretreatment of Fe-Cr alloys has a strong effect on the scale morphology and subsequent oxidation rate For instance, Caplan indicated that several Fe-Cr alloys show improvement in the corrosion resistance due to cold work, with greater than 16% Cr required to show the optimum benefit. Khanna and Gnanamoorthy examined the effect of cold work on 2.25%Cr-l%Mo steels at temperatures between 400°C and 950°C over 4h in 1 atm O2. They found that up to 90% reduction by cold rolling had a negligible effect on the oxidation rate up to 700°C. However, above 700°C there was a general reduction in the kinetics... 

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 ... 

The Effect of Adhesive Primers. In practice, adhesive bonds involving metal adherends often use primers as pretreatments of the metal surface prior to bonding. Table IV shows the durability of composite-metal bonds prepared with adhesive C over a series of primers (of varying corrosion resistance) in 240 hour salt spray test. The results indicate that the performance of bonds is directly related to the corrosion resistance of the primer used to prepare the adherend surface. In general, the adhesion of the primer to the steel adherend, rather than the adhesive chemistry. 

A large segment of the metal parts produced by industry are painted for both decorative purposes as well as to increase the corrosion resistance and extend the useful life of the product. To obtain maximum quality from painted metal articles, it is of paramount importance to pretreat the metal parts with a conversion coating process. ( 1,2) Pretreatment processes contribute a significant improvement in corrosion protection and durability to metal articles by ... 

Surface Morphology. The initial Integrity of an adhesively bonded system depends on the surface oxide porosity and microscopic roughness features resulting from etching or anodization pretreatments. (17) The SAA surface characterized in this study consists of a thick (9 ym), porous columnar layer which provides excellent corrosion resistance in both humid and aggressive (i.e., Cl ) media. I The thinner FPL oxide provides a suitable substrate surface for evaluating the candidate inhibitors. 

The shell-side steam does not demand any special corrosion-resistance properties from the material. The steam has already been deaerated and deionized during pretreatment. The maximum anticipated temperature is only 380°C. This permits the choice of simple carbon steel for the exchanger shell. 

Results from both studies indicate that various pretreatments of alloys can enhance the corrosion resistance of the materials. 

Titanates are valuable in other paint applications. Corrosion-resistant coatings have been described for tinplate, steel, and aluminum (440—444). Incorporation of phosphoric acid or polyphosphates enhances the corrosion resistance. Because titanates promote hardening of epoxy resins, they are often used in epoxy-based paint (445). Silicones (polysiloxanes) are often cured by titanates. Pigments, eg, Ti02, Si02, Al O and Zr02, are frequently pretreated with titanates before incorporation into paints (441,446). In these applications, the Ti(OR)4 compounds are often mixed with Si(OR)4, Al(OR)3, Zr(OR)4, and other metal alkoxides (12). 

The image of scanned surface of corrosion test specimens as well as calculated corrosion area and corrosion width can be used to see the influence of the interface on the corrosion resistance of the overall system. Methods of pretreatment of substrate sheet, plasma conditions, and the sample identification codes are shown in Table 28.1. 

Figure 31.14 shows the Rp values of [2A] with three different chemical pretreatments and with a TMS plasma polymer on each of the three pretreated surfaces, as well as on the control [2A]CC surfaces (chromate conversion-coated 2A). It can be seen that the Rp values of [2A] were decreased to some extent by pretreatment of alkaline cleaning and were drastically reduced by alkaline cleaning plus deoxidization. As observed in the XPS results, the accumulation of Cu elements and removal of oxide layer on [2A] surfaces were presumed responsible for the reduction in corrosion resistance of these chemically pretreated [2A] panels. 

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. 

Pure magnesium substrates were chemically pretreated at 70 °C with a Ca/Na phosphate solution to obtain a porous brushite layer and subsequently PEO at a voltage of 450 V, a frequency of 1000 Hz and a duty cycle of 40% to deposit strontium-containing hydroxyapatite (Lu et al., 2014). Figure 7.59 shows that the Sr-hydroxyapatite coating provided improved corrosion resistance in HBSS to the pure Mg substrate. The characteristic corrosion parameters are presented in Table 7.6. 

Pretreatment is one of the most expensive and least technologically mature steps in the proeess for converting biomass to fermentable sugars [4]. Costs are due to the use of steam and chemical products and the need for expensive corrosion resistant reactors however, pretreatment also has great potential for efficiency improvement and lowering of costs through research and development [5-8],... 

Magnesium CCCs are usually formed in acidic solutions in which Mg surfaces are easily activated. Nitric acid-and chromic acid-based mixtures are common. Because of the high corrosion susceptibility of Mg alloys, CCCs are more often used as a surface pretreatment for paint than as stand-alone corrosion-resistant coatings. 

For aluminium, the choice of pre-treatment will depend on the aluminium (or the aluminium alloy) itself and the corrosion resistance required. Chromium-containing pretreatments have proved to be the most effective, but concern in recent years over the toxicity associated with the chemicals used is now driving forward the development of... 

Steel. Steel surfaces are cleaned to remove oily soils and, if necessary, pickled in acid to remove rust. Clean steel is generally pretreated with phosphates to provide corrosion resistance. Other pretreatments for steel are chromates and wash primers. 

The main phosphate pretreatments for steel are iron phosphate and zinc phosphate types. Zinc phosphates are preferred for best corrosion resistance out-of-doors. 

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