Steel pretreatment

Yet another option for domestic appliances is to make the appliance from precoated coil. The appliance has to be designed to minimise the problem of unprotected cut edges. Electroplated zinc-coated steel, pretreated, primed and finished with special polyester-melamine, is used. The finish is designed to be hard at room temperature, yet accept bending and forming, probably, but not necessarily, at somewhat higher temperatures (ca. 60°C). 

On sheet iron a groundcoat, including cobalt and nickel, is generally used, but for mass production (e.g. cookers) use of decarbonised steel and direct application of colours is more common. This involves a more complex steel pretreatment. 

Figure 5. Epoxy/steel joint lap shear strength as a function of 57°C water immersion time for the following steel pretreatments (—0—1 citric acid treatment (— —) citric acid treatment followed by...

Donaldson, J.D. and Grimes, S.M., 1987, Scale prevention in steel pretreatment by magnetic treatment. Steel Times Int. 44 - 45, December. 

Fedrizzi L, Rodriguez FJ, Rossi S, Deflorian F, Di Maggio R (2001) The use of electrochemical techniques to study the corrosion behaviour of organic coatings on steel pretreated with sol-gel zirconia films. Electrochim Acta 46(24—25) 3715-3724... 

To test the anticorrosion behavior of polymer films prepared in nitrate medium using mild steel pretreated widi dilute nitric add, we synthesized simple PPy films (referred to as (PPy)g) and modified PPy films (referred to as (PPy)m). (P )m is obtained by copolymerization of a 5 0.5 0.5 mixture of pyrrole, N-mediylpyrrole-2-carboi c add aM N-mediyl-2[0d laniino]pyrrole (total concentration 0. IM). The smictuies of these monomers are shown in scheme 1... 

Where experience shows that specific steels, pretreatments, thermal and mechanical treatments, pickling and hot dip galvanizing procedures have been satisfactory, the information serves as an indication that an embrittlement problem is not to be expected for the same combination of steels, pretreatments, thermal and mechanical treatments, and galvanizing procedures. 

Figure 9-35. AFM image of ihe surface morphology of carbon steel pretreated in neutral 0.5 mol dm NaCIOi solution in the presence of a) 3xI0 mol dm- HEDP and b)3xl0 mol dm- HEDP+ 6x lO" mol dm Zn (Fel-hdsi et al., 1999a). (Reproduced by permission.) Re K) real part of impedance,...

Stainless steel pretreatment Surface structure modifica- Makkus et al. (2000)... 

Montemor M. R, Cabral A. M., Zheludkevich M. L. and Ferreira M. G. S. (2006a), The corrosion resistance of hot dip galvanized steel pretreated with bis-functional silanes modified with microsilica . Surf. Coat. TechnoL, 200,2875-85. 

Substrate Properties. It is clear from equation 5 that higher hardness of the substrate lowers friction. Wear rate of the film also is generally lower. Phosphate undercoats on steel considerably improve wear life of bonded coatings by providing a porous surface which holds reserve lubricant. The same is tme for surfaces that are vapor- or sandblasted prior to appHcation of the soHd-film lubricant. A number of typical surface pretreatments are given in Table 13 to prepare a surface for solid-film bonding (61). 

Alkaline permanganate pretreatment of steel for the removal of heat scale and smut prior to acid pickling results in faster descaling and reduced metal attack (see Metal surface treatments Metal treatments). Stainless steel alloys can also be cleaned by alkaline permanganate followed by pickling in nonoxidi2ing acids (260). 

Foulants enter a cooling system with makeup water, airborne contamination, process leaks, and corrosion. Most potential foulants enter with makeup water as particulate matter, such as clay, sdt, and iron oxides. Insoluble aluminum and iron hydroxides enter a system from makeup water pretreatment operations. Some well waters contain high levels of soluble ferrous iron that is later oxidized to ferric iron by dissolved oxygen in the recirculating cooling water. Because it is insoluble, the ferric iron precipitates. The steel corrosion process is also a source of ferrous iron and, consequendy, contributes to fouling. 

In addition, the magnetic characteristics of a material can change as a function of stress (e.g., unannealed series 316 stainless steel can be magnetic after machining), temperature, pressure, and physical and chemical treatment. Therefore, when two paramagnetic materials with similar magnetic susceptibilities are to be separated, the possibility that pretreatment will facilitate subsequent separation should be studied. 

The protection of steel surfaces by paint depends significantly on the chemistry of the paint-metal interface. The system has many variables, because the metal surface is usually pretreated in a variety of ways, including galvanizing and phosphating. Indeed, there are probably several interfaces of importance, and corrosion protection might be a function of the conditions at all of them. 

Many types of chemical treatment are used in industry. Chromic, permanganic, sulphuric, and chlorosul-phonic acids are often used as the oxidants. It has been shown that the adhesion of polyethylene to substrates, such as cellophane, steel, aluminium, and epoxy adhesives, improves upon pretreatment with any of the etchants mentioned previously. 

These materials are now widely used for coating both steel and concrete surfaces that are subject to a particularly aggressive environment (e.g. North Sea oil platforms). There is less validity for their use under normal atmospheric conditions since they are relatively expensive and tend to chalk on exposure to sunlight. However, their use as zinc phosphate, pretreatment or blast primers for blast-cleaned steel which is subsequently overcoated by any other paint system is an extremely valuable contribution to the painting of new steel work. 

The presence of small quantities of S in steels has little effect on the initial scaling rates in air, but may be detrimental to long-term scale adhesion. Sulphur has, however, been shown to be detrimental to breakaway oxidation in CO/CO2 environments. However, sulphur has been shown to reduce the total uptake of carbon in the steel under CO/C02 and reduce the scale thickening rate. In this context, free-cutting steels were found to oxidise at a significantly lower rate, as did steels subjected to pretreatment in H2S. 

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... 

Before steel strip or rod can be cold rolled, tinned, galvanised, or enamelled, etc. any scale formed on it by previous heat treatment must be removed. This can be done by mechanical and other special methods, but if a perfectly clean surface is to be produced, acid pickling is preferred, either alone or in conjunction with other pretreatment processes. 

The processes are dealt with fully in Chapters 11, 14 and 15. Because many paint systems include an initial surface pretreatment, e.g. chromated aluminium or phosphated steel, BS4479 1990, Part 3 deals with conversion coatings and should be consulted by designers. Whatever the method of treatment, liquids must be able to drain quickly and freely from the surfaces. Crevices where liquids can become entrapped are best avoided. The surface configuration needs to be such that active solutions can be washed away, leaving the surface to be painted completely free from unreacted pretreatment solution. Failure to achieve the requisite level of freedom from the surplus chemicals causes paint failure, e.g. osmotic blistering. 

Aluminium coatings also provide a suitable key or pretreatment for subsequent coatings, e.g. aluminised steel provides a good base for vitreous enamel. 

Etch primers are widely used. They are mostly based on polyvinyl butyral and contain chromates and phosphoric acid. They are said to act both as primers and as etching solutions because it is believed that the chromates and phosphoric acid form an inorganic him, which provides adhesion, while oxidised polyvinyl butyral provides an organic him. For direct application to new galvanised steel, the best known primers are based on calcium orthoplumbate pigment and metallic lead, but these are now less used for environmental reasons. Zinc-dust paints and zinc-phosphate pigmented paints are also used, but the trend is to use pretreatments to assure good adhesion. 

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