In the last article, the article talks much on the specific operation methods for the surface treatment of steel pipe, and in this article, which problems should be pay attention on when using spray (cast) shot derusting will be the main topic.

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Descaling grade
The construction process commonly used for anti-corrosive coatings of the steel tube like vinyl, epoxy phenolic always require the steel tube surface nearly white level (Sa2.5). And the practice evidence has proved that using this descaling grade material can remove almost all of the scale, rust and other debris, and the anchor pattern depth can reach 40 ~ 100 microns, and fully meet the requirements of anti-corrosion layer and steel adhesion, and shot derusting spray (cast) process can be used with low running cost and stable and reliable quality level to nearly white (Sa2.5) technical conditions.
Spray shot abrasive
In order to achieve ideal cleaning effect, the abrasive should be selected according to the degree of hardness on the surface of the steel tube, the original corrosion, requirements of surface roughness, coating types, etc. For single layer, layer or three layer PE coating epoxy, using a mixture of steel shot and steel grit grinding are more likely to achieve ideal cleaning effect. Steel pill have the effect of the reinforcement steel surface, and it has steel grit etching on the surface of the steel. Steel shot and steel grit mixed abrasive (usually for 40 ~ 50 HRC hardness steel shot, steel grit of hardness of 50 ~ 60 HRC can be used for all kinds of steel surface, even in the class C and D corrosion of steel surface, cleaning effect is very good also.
Abrasive particle size and proportion
In order to obtain better uniform cleanliness and roughness distribution, ratio of abrasive particle size and matching is quite important. Too much roughness may cause the thinner anchor lines; at the same time, as the anchor lines too deep, in the process of anti-corrosion coating, it is easy to form bubbles, serious impact on the performance of anti-corrosive coating.
If it is not rough enough, it will cause the loss of coating adhesion and impact resistance strength. For serious internal pitting, it can not only depend on the high-strength of the larger particles, but also reply small particles to polish away the corrosion products to achieve clean effect. At the same time, the reasonable proportioning design not only can slow the abrasive wear of pipeline and spray nozzle (blade), and the utilization of abrasive can be greatly improved. Usually, the particle size of steel shot is 0.8 ~ 1.3 mm, steel grit size is 0.4 ~ 1.0 mm, with 0.5 ~ 1.0 mm as main ingredients. The ratio of sand and shot is average 5 ~ 8.
It should be noted that in practice, the ideal ratio of steel shot and steel grit is very difficult to achieve, the reason is that the harder and brittle steel shot is easily been broken than steel grit. So in the process the sampling test should be continuously went on. According to the particle size distribution, adding new abrasive cleaning machine, and mixing in a new abrasive, the number of steel shot to account for the main.
Rust removal rate
Steel tube derusting speed depends on the type of abrasive and abrasive displacement, generally should choose low attrition rate of abrasive, so it will improve speed and long blade life.
Cleaning and preheating
Before spray (cast) shoot processing, adopt the method of cleaning to remove oil on the surface of the steel tube, the heating furnace for preheating should be 60 to 40 ℃, in order to support the steel pipe surface in a dry station. In spray (cast) processing, due to steel tube surface without oil and dirt, the efforts of cleaning can strengthen. The drying state of the surface of the steel tube also is helpful for steel shot, steel grit and the separation of rust and scale, make the steel tube surface clearer.
Emphasis on the importance of surface treatment in the production, strictly control the descaling process parameters. In actual construction, the steel pipe anti-corrosion layer of the peel strength value is exceed much over the standard requirements, which ensure the quality of the anti-corrosive coating,improve the technological level and reduce the production cost.