by Ron Joseph
Selecting a Protective Coating System
Q. A mild steel machine part is to be coated for corrosion prevention. The part is exposed to (discontinuous) splash of industrial cooling water. What will be a suitable coating system for this?
A. Without knowing more about the part or the conditions of exposure, I suggest you look into epoxy systems; i.e. epoxy primer and epoxy topcoat. On the other hand, if the part will be exposed to sunlight, the epoxy will chalk and loose its gloss. But if you are not concerned with it appearance, then the gloss issue will not affect the performance of the coating.
Q2. Thanks a lot for your really quick response.
The corroding substrates are in fact, exposed steel parts of glass drilling machines (which are located inside the industrial plant.) The environment is mainly water used as cooling agent during the drilling process, which comes in contact with the parts. (along with the glass powder that has been drilled out)
The water has been characterized as (approx) having
1) pH- 6.5 to 7.5
2) Total Dissolved Salts <350 ppm
3) Hardness <50ppm
4) Chlorides <50ppm
I hope this gives a clearer picture of the case at hand. What about the choice (or necessity) of pigments used in the primer (is any pigment preferred over the others), will it be of any significance? As of now, the substrates are corroded to some extent ... so what extent of surface preparation will be necessary for application of the coating?
A2. The epoxy system is still approppriate for this application. I suggest you go to the website of a company such as www.carboline.com and select the products you wish to use. The epoxy can either be zinc-rich, or zinc chromate. In the US we are restricting the use of zinc chromate because it is a carcinogen, but this primer is still available. The zinc-rich primer is more difficult to apply.
For a zinc-rich primer you should abrasive blast the metal to a near-white, or preferrably white metal finish. A zinc chromate primer is probably more forgiving of surface preparation, but you can expect to sacrifice service life. The better the preparation, the longer the coating system will perform.