Читать книгу Corrosion Policy Decision Making - Группа авторов - Страница 34

One of the most important phases of a paint operation is the surface preparation, which sometimes costs quite a bit. Appropriate weather conditions, good blasting materials, equipment, and proper technical skills are needed to prepare the surface. Most of the paint problems of industrial equipment are due to the weakness of the surface preparation. Some examples of these problems are observed during the field operation inspections seen in Figure 2.16. In scientific documents, special attention is referred to this issue [11].

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.

Preparation methods	Adhesion (MPa)ASTM D4541	Disbanding condition
Washing with white sprite	1.73	Paint layer from metal surface
Wire brush ST3	3.19	Paint layer from metal surface
Weak acid washing	2.89	Paint layer from metal surface
Hard acid washing	4.48	Paint layer from metal surface
Phosphating	4.30	Paint layer from metal surface
Wet sand blast, water blast	8	Paint layer from metal surface
Dry sand blast Sa3	11.63	Paint layer from metal surface

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.

Test	Standard	Result for all samples
Hardness (mohz)	SSPC‐AB‐1	At least 6
Weight change on ignition	SSPC‐AB‐1	Max. 5%
Oil content	ASTM D‐7393	No visible
Specific gravity	ASTM C‐128	Min. 2.5
Moisture content	ASTM C‐566	0.5%<
Water soluble contaminants	ASTM D‐4940	<1000 μs/cm

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

Abrasives	Adhesion test before exposureASTM D4541Psia	Adhesion test; 1500 hours salt spray and 24 hours dryingASTM D4541 Psia	Adhesion test; 1500 hours humidity and24 hours dryingASTM D4541Psia
Copper slag Sample A	1687	528	1603
Copper slag Sample B	1541	486	Test error
Copper slag Sample C	1569	388	1160
Copper slag Sample D	1446	516	1223
Cupper slag Sample E	1355	422	1306
Supper slag Sample F	1525	1329	1385
Silica sand Sample G	1687	1855	989
Silica sand Sample H	1900	1917	1898

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.