Steel surface preparation

Surface Preparation. Steel hot rolled at 800-900 °C acquires a tenacious oxide layer (mill scale) that is cathodic with respect to the steel to the extent of about 300 mV. In the presence of an electrolyte (seawater containing 3.5% salts, mainly sodium chloride) the steel would corrode and pit and roughen severely. The first process in new construction and refurbishment is therefore the complete removal of mill scale. A small amount of very light-gauge steel is prepared by acid pickling, but most steel for ship and off-shore construction is centrifugally or pneumatically blasted with steel shot that can be recycled or with expendable abrasive grit. Freedom from scale and soluble salt contamination are the main requirements, texture and profile are less important [11.13],... 

Boron trifluoride has been used in mixtures to prepare boride surfaces on steel (qv) and other metals, and as a lubricant for casting steel (see... 

In Section 4.3 several recently developed and experimental surface preparation methods for steel such as conversion coatings, plasma spray and sol-gel that attempt to simultaneously improve durability and bond strength over grit blasting will be discussed. 

Direct bonding. In many high-volume production applications (i.e., the automotive and appliance industries), elaborate surface preparation of steel ad-herends is undesirable or impossible. Thus, there has been widespread interest in bonding directly to steel coil surfaces that contain various protective oils [55,56,113-116], Debski et al. proposed that epoxy adhesives, particularly those curing at high temperatures, could form suitable bonds to oily steel surfaces by two mechanisms (1) thermodynamic displacement of the oil from the steel surface, and (2) absorption of the oil into the bulk adhesives [55,56]. The relative importance of these two mechanisms depends on the polarity of the oil and the surface area/volume ratio of the adhesive (which can be affected by adherend surface roughness). 

Despite the progress outlined in this chapter, much work remains to be done in the metal surface preparation arena. For example, there is still no ideal surface preparation method that does for steel what anodization processes do for aluminum and titanium. The plasma spray process looks encouraging but because it is slow for large areas and requires rather expensive robot controlled plasma spray equipment, its use will probably be limited to some rather special applications. For more general use, the sol-gel process has potential if future studies confirm recently reported results. 

Application of protective paints consists of surface preparation of steel, priming coat and finishing coats. Wherever possible, steel should be blast-cleaned before painting. Primers thoroughly wet the metal to promote adhesion of finishing paints and carry inhibitive pigments. For example, red lead oxide will minimize the spread of rust on metal surfaces. The total thickness of fmishing coats must be at least 0.125 mm for adequate protection and life. Four coats of paint usually are necessary to achieve this. 

No. 1 Pictorial Surface Preparation Standards for Paindng Steel Structures... 

Surface preparation is of prime importance, and optimum performance of modern protection coatings can be achieved only if the surface of the steel has been adequately treated. The method of surface preparation depends on the shape and size of the structure or component. Thus it is preferable to blast-clean an openwork steel structure by manual methods, since with this type of structure automatic blast cleaning would lead to excessive impingement of the abrasive on the machine itself. 

After suitable surface preparation, e.g. blast cleaning, low-alloy steels can be coated by paints, sprayed metal coatings, etc. and there is some evidence that such coatings last longer than on mild steel under similar conditions of exposure. ... 

Svensk Standard SIS 055900-1967, Pictorial Surface Preparation Standards for Painting Steel Surfaces. 

The anode material must stay firmly attached to the steel insert, which is necessary to conduct the current from the anode to the structure, throughout its design life to remain effective. Consequently surface preparation (by dry blast cleaning ) of the insert prior to casting, to ensure a sound bond with the anode material, is essential. Voids at the insert/anode material interface are undesirable as these will also affect the bond integrity. 

The possible methods of surface preparation before painting hot rolled steel are discussed in the following sections. 

Very high standards of surface preparation are necessary, e.g. abrasive blast cleaning to Sa2y, BS7079 Part Al, 1989 for steel components... 

Deposition by metal spraying can also be used for the reclamation of worn parts in this case, surface preparation is often accomplished by machining, i.e. by cutting a rough thread on the surface or by increasing the surface area of the part by grooving. Such methods. are not, however, normally used in corrosion prevention, except in the case of pump rods, which can be built up with nickel or stainless steel. 

The outstanding virtue of zinc-rich paints is simplicity in application. No special equipment is required and the operation can, of course, be carried out on site, large or small structures being equally suitable for treatment. While there is some evidence that the zinc-rich paints will reduce iron oxides remaining on the steel surface, proper surface preparation is as important here as with traditional paints if the best results are to be achieved. The main use of zinc-rich paints is to protect structural steel-work, ships hulls, and vulnerable parts of car bodies, and to repair damage to other zinc coatings. 

Penetration of rust through an otherwise intact paint film is usually a result of inadequate surface preparation before painting, especially over weathered and hand-cleaned steel. However, superficial rust staining may be traceable to dissolved iron salts, e.g. in bilge water from a ship s deck. 

Photographic standards and written descriptions of various stages of visual cleanness of steel surfaces after surface preparation by blasting are available in British Standard 7079 Part A1.1989, Series Sa. ... 

Pickling as a method of surface preparation is generally carried out by immersing the steel in an acid bath and then rinsing with clean water. It is essentially a works process because it must be carefully controlled. Site application of acid washes, etc, is not recommended. 

Nature of the metal surface Clean, smooth, metal surfaces usually require a lower concentration of inhibitor for protection than do rough or dirty surfaces. Relative figures for minimum concentrations of benzoate, chromate and nitrite necessary to inhibit the corrosion of mild steel with various types of surface finish have been given in a recent laboratory studyThese results show that benzoate effectiveness is particularly susceptible to surface preparation. It is unwise, therefore, to apply results obtained in laboratory studies with one type of metal surface preparation to other surfaces in practical conditions. The presence of oil, grease or corrosion products on metal surfaces will also affect the concentration of inhibitor required with the... 

Nature of the metal surface The critical concentration of an anion required to inhibit the corrosion of iron may increase with increasing surface roughness. Thus, Brasher and Mercer" showed that the minimum concentration of benzoate required to protect a grit-blasted steel surface was about 100 times greater than that required to protect an abraded surface. However, surface preparation had little effect on the critical inhibitive concentrations for chromate" or nitrite " The time of exposure of the iron surface to air after preparation and before immersion may also affect the ease of inhibition by anions. There is evidence """ that the inhibition by anions occurs more readily as the time of pre-exposure to air increases. Similarly, if an iron specimen is immersed for some time in a protective solution of an inhibitive anion, it may then be transferred without loss of inhibition to a solution of the anion containing much less than the critical inhibitive concentration . ... 

Surface preparation before measurement is important, as sound travels about twice as fast in steel as in paint. Many years accumulation of paint can indicate a thicker steel wall than actually exists. 

Certain British Standards, e.g. for galvanising or metal spraying", contain detailed sections on surface preparation. Others are concerned with acid pickling but the chief method of preparing iron and steel for a long-life protective system is by blast cleaning see Section 12.4). 

It frequently happens that after surface preparation shelling or other imperfections in the steel surface are revealed. It is preferable to deal with these by grinding or filling as necessary before any other coating is applied. 

A wide range of paints and other organic coatings is used for the protection of mild steel structures. Paints are used mainly for protection from atmospheric corrosion. Special chemically resistant paints have been developed for use on chemical process equipment. Chlorinated rubber paints and epoxy-based paints are used. In the application of paints and other coatings, good surface preparation is essential to ensure good adhesion of the paint film or coating. 

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