Steel surface, blasted

The wash primer is a special type of vinyl coating. This material contains a poly(vinyl butyral) resin, zinc chromate, and phosphoric acid in an alcohol-water solvent. The coating is so thin it is HteraUy washed onto a freshly blasted steel surface, where it passivates the metal surface by converting it to a thin iron phosphate-chromate coating. The alcohol solvent makes it possible to apply the coating over damp surfaces. The coating forms the first coat of... 

In BS 4232, Surface Finish of Blast Cleaned Steel for Painting, first quality corresponds to SA3,2nd to SA2-5 and 3rd toSA2of the much more extensive Swedish Standards Commission s SIS 055 900-1962, RusI Grades for Steel Surfaces and Preparation Grades Prior to Protective Coaling, Stockholm (1962) see also BS 7079 Part A1 (1989)... 

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. ... 

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 . ... 

Figure 19.60 illustrates a steel surface which should have been blast cleaned to a high standard before being coated at works with a zinc-rich epoxy primer and on site with two-pack intermediate and finishing coats. After exposure for 18 months in a marine environment, flaking millscale from beneath the paint was observed, and a survey showed that the paint... 

Steel surfaces do not provide any unusual difficulties during coating operations, as long as the steel is of high quality and the environmental conditions are strictly controlled. Specifications usually call for an abrasive blast that meets Steel Structures Painting Council (SSPC) Spec. 5 ("white metal") finish although steel to be used in non-nuclear areas usually receives an SSPC 10 ("near white") finish or an SSPC 6 ("commercial") finish. Areas that require special attention are welds, seams, edges, channels, etc. 

Sa3 Blast cleaning to visually clean steel. Surface should be completely free from oil, grease, mill scale and rust... 

In galvanized steel constructions, blasting is also necessary to provide a coatable surface. The sweep-blasting method is used in which the zinc surface is roughened without removing a significant amount of the zinc layer. When constructions made of aluminum alloys are coated, their surfaces must be blasted with iron-free blasting materials. 

Priming. In new construction the freshly blasted steel surface is highly reactive and requires corrosion protection during construction. It immediately passes through an... 

In order to install cathodic protection, continuity can be established by welding in extra rebars. However, at Florida DOT one approach has been to expose bars in damaged areas, grit blast them clean and apply arc sprayed zinc directly onto the steel and then across the steel surface. This provides galvanizing directly on the steel and SACP to the steel embedded in the concrete. Multiple continuity connections are established by the sprayed zinc. 

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