Corrosion Policy Decision Making

Dry abrasion blasting with silica sand, copper slag, steel grits, etc. is the most correct and common method of field and workshop surface preparation. In a scientific research, the author investigated the surface preparation factors affecting the primer adhesion to steel substrate; the primer was an epoxy zinc phosphate with a dry thickness of 60 μm. The surface was prepared by different methods. The results are according to Table 2.1. Primer adhesion on all samples was acceptable, however, there was a noticeable difference between the adhesions of the primer to the steel surface prepared by different methods. Between some common methods of surface preparation, abrasive blasting especially by sand, to the degree of cleanness of Sa3, shows the highest quality on the paint adhesion.

Figure 2.15 Examples of unqualified workers.

Figure 2.16 Some weakness of the surface preparation. Paint application on rusted surface (left). Unsuitable abrasives and reuse of them several times (right). Weak surface preparation on weld lines (down).

Table 2.1 Adhesion of a common zinc phosphate epoxy primer on the steel surfaces prepared with several methods.

In other research, the effect of different types of abrasives on steel preparation was investigated. Physical and chemical properties of tested abrasives are discussed in [17–19].

Properties of some abrasives are determined according to Table 2.2. In a series of tests, the author examined and compared the adhesion of a zinc phosphate epoxy primer with the same thickness on steel sheets that were prepared with these different abrasives. Results are summarized in Table 2.3.

Often, the functional performance of abrasives, such as the effect on primer adhesion is very important. It affects the mechanical stability of the film, as well as its chemical resistance, especially when the film is exposed to humid and saline environments during service time. The author investigated the effect of a number of abrasives on cured film performance by doing several tests. For that, some samples were blasted by different abrasives. They were coated with the same paint, having nearly dry film thickness. Then they were tested in a humidity chamber according to ASTM D2247, and in a salt spray chamber according to ASTM B117 [20, 21].

In these experiments, silica sand compared with different abrasives used for blasting surface. A noteworthy point about carbon steel samples prepared with copper slags was that their particles trapped in all samples. Slag particles were observed which could not washed out even by using air pressure (five bar pressure) and washing with solvent. However, for samples prepared with silica sand, leaving excess particles from the surface with compressed air were easily and completely removed from the surface. All samples were coated with the same paint layer with a similar dry film thickness.

Table 2.2 Properties of tested abrasives in blasting.

Table 2.3 Effect of abrasives on adhesion of primer at humid and salty environments.

As seen in the Table 2.3, after complete curing and before beginning the corrosive tests, the adhesion of all samples was in acceptable range. Separation of all samples were with an interlayer that did not return to the abrasive quality. The sheets were prepared with different abrasives, placed and tested for resistance in a humidity cabin environment at 100% humidity, and similarly in a salt spray chamber for 1500 hours, separately. The results were interesting; the samples prepared with abrasive silica sand showed a significantly better resistance than others. In addition, they had more strength adhesion at the end of the laboratory exposure tests. The results are shown according to Table 2.3. Therefore, type, shape, and size of the abrasive material have a significant effect on long life of paints in the abrasive blasting method.

Painting application requires special skills; the inexpert technician can lead to the destruction and complication of the film. Examples of paint application problems are shown in Figure 2.17. It is necessary for the executors be aware of the causes of paint defects and the method of avoiding, eliminating, and correcting these complications.

Figure 2.17 Some examples of paint application problems. Rusting on primer due to thin primer layer (a). Running of paint and settling of dirt due to adjacent blasting operation (b).

Many paint defects are mainly related to the method of their application. The wrong choice of hardener and thinner for paints, incorrect mixing of paint components, and wrong choice of different layers of a paint system are the most common causes of complication in paint application. Accuracy at the important points in the catalog can cut problems. Lifting, orange peel, overspray, pinholes, spatter coat, etc. are some of the complications that may occur due to these problems [7].

Excessive hot or cold temperature of the substrate, high humidity, dust, and local pollution are other factors that may cause complications during paint application. Adherence to implementation instructions greatly reduces these problems. Complications such as amine blushing, blushing, holidays, rusting, etc. are observed in these conditions [7].