Do we need phosphating after sand blasting followed by powder top coat on hot rolled steel components?
No. It is not necessary if the salt spray resistance required is less than 550 hours. The purpose of sand blasting is to improve the surface profile of the component with a surface free from rust and heavy scales. Yet, the surface is not ready for powder coating if it contains oil and grease even after sand blasting.
One cannot expect complete removal of oil and grease just by sand blasting. It removes scales and rusts only. The oily components should be cleaned with some cleaning thinner or kerosene or MTO prior to sand blasting. This will help in preventing the agglomeration of sand or grit particles. Once sand blasted, the components need cleaning with solvent or thinners again. When the sand blasted and solvent cleaned surface is totally free from oil, grease, rust, scale and dust in any form, it is ready for powder coating.
How does one select a particular type metal pretreatment?
A job coater can expect the sheet metal components with the following surface conditions (Fig.1).
A) Thick oily, greasy, rusted steel with scales – In this case, sand blasting with or without metal pretreatment is recommended based on the salt spray resistance hours required. However, if the component is found to contain tool marks due to cutting or laser cutting and welding, that area should be smoothly buffed prior to sand blasting.
NOTE: Area with welding joints (Fig. 2) is more prone to electrochemical corrosion. It needs to be thoroughly buffed. That surface should be thoroughly cleaned with suitable solvent if there is any silicone based anti-spattering agent has been used.
B) Mild oily, rusted steel – 7 Stage phosphating followed by powder coating
C) Oily steel sheet with no trace of rust – Nano zirconia coating followed by powder coating. Nano zirconia coating serves the purpose of surface protection from corrosion and humidity after powder coating. This does the job similar to phosphating. This kind of coating enhances the salt spray resistance even after 600 hours. The greatest advantage of this Nanocoating is that there is no sludge formation and it can be coated at room temperature within 2 minutes.
Finally, one can select any kind of metal pretreatment based on the surface condition of the incoming sheet metal components. Yet, if the metal pretreatment is not done properly, the faulty coating by itself is detrimental to the performance of the top coat.
Suitable recommendations of finishing for heavy oiled and HR components with scales
Method – 1 (for Salt spray resistance < 600 h)
Solvent dip and cleaning -> Sand blasting -> solvent cleaning -> air blow. Make sure the sand blasted surface (Fig. 3) is totally free from rust, dust in any form or oil or grease before taking up for final coat of polyester epoxy powder followed by curing at 230o C.
Method – 2 (for Salt spray resistance > 600 h)
Solvent dip and cleaning -> Sand blasting -> solvent cleaning -> air blow -> Pre-degreasing -> alkaline degreasing -> Water rinse 1 -> Water rinse -2 -> Surface conditioning -> Phosphating -> DM water rinse-1 -> Fresh DM water rinse – 2 -> Pressurised air blow to drive water drops trapped in silent corners -> Water dry off oven at 110 o C. The phosphate coated surface should be free from water, oil, rust, or dust in any form (Fig.4). Within the shortest interval of time the polyester epoxy powder coat should be given followed by curing at 230 o C. Such type of powder coat with a perfect pretreatment will surely pass on cross hatch test (Fig. 5).
There should be always a clear understanding between the metal fabricator and job coater regarding the type of rust preventive oil used on the fabricated components and job coater should have a suitable cleaning thinner to remove the rust preventive oil. The interval between sand blasting or pretreatment and powder coating should be less than an hour in a dry and dust free atmosphere.