Q: We currently are planning on using cold-rolled 304 stainless square tubing for our instrument housings, and the tubing comes slightly oxidized and slightly pitted. We are looking to provide the customer with a finish for the housing that looks professional and prevents it from going back to the way we received it.
We were looking at possibly painting, powder coating, polishing, or plating the housing, but I am leaning toward powder coating as a cost-effective means to protect the housing. The environment the instrument will be in ranges from -40°C to +100°C and is occasionally dusty, oily, and near sea water (not touching or emerged). Given that the tubing is 304 stainless steel, I know adhesion is the big problem.
Can you recommend the proper way to prepare the surface of a 304 stainless steel material? Details on the type of degreaser to the type of grit (non-ferrous) or chemical (i.e., silane), to the primer type so that UV and sea salt corrosion will not affect the housing for up to 10 years would be greatly appreciated.
A: This is an excellent question, and it is very wise to explore all your options before committing to a new process. Powder coatings are a very good finish for stainless steel if the process is designed and maintained properly. The key is in the cleaning and metal pretreatment process that precedes application of the powder.
The process you should consider for powder coating starts with media blasting. A high-quality medium such as aluminum oxide will suffice. Follow the blasting with a well-controlled alkaline degreasing step to remove any residual oils, process fluids, and handling contaminants. It is also important to keep your blast media clean.
Next, I suggest a passivation step to remove any iron-based residues on the stainless surface, as well as the possible presence of residual sulfides. Trace amounts of either contaminant can act as sites for subsequent corrosion. Passivation of stainless steel typically involves immersing the part in a nitric acid bath, although nitric acid/sodium dichromate or citric acid solutions can also be used. The concentration of acid can range from 20-50% by volume. ASTM A380 describes standard practices for passivating stainless steel.
After thoroughly drying the parts, it is wise to powder coat the passivated surface as soon as possible. Chemically pretreated surfaces have a finite shelf life before they begin to degrade. Using a high-quality powder coating precludes the need for a primer coating.
Meeting the durability required in the environments you describe requires the powder coating to be applied at a reasonable thickness as specified by the powder coating supplier; 65-80 microns are usually sufficient for an outdoor application. It is also critical to ensure that the coated parts see enough temperature and time to complete the cure to the powder coating chemistry. When a powder supplier quotes a recommended bake, they are referring to the time at or above the stated temperature that the part has reached. It is therefore important to factor in the heat-up time needed to reach the bake temperature.
As for ensuring acceptable coating performance in the field, I can recommend a couple of practical tests. I suggest that you prepare a few test pieces that have been run through the entire cleaning/passivation/coating/curing process and perform the following tests:
1. Simple “X” crosscut dry adhesion: Cut a 40-mm by 40-mm “X” into the coating with a sharp blade. Apply a strong tape such as 3M 2517 or Permacel #99 and pull sharply. The coating should exhibit no loss of adhesion. A similar test can be done with a “crosshatch” as described in ASTM D3359. This entails a perpendicular series of cuts that create a grid. Nevertheless, the simple “X” crosscut test is usually sufficient to determine if adhesion is acceptable.
2. Hot water immersion adhesion: Place the coated samples evaluated in the adhesion test described above into a 75°C water bath for 24 hrs. Remove and allow them to dry, then recheck tape adhesion. The coating should show little or no loss of adhesion.
3. I would confirm these test results with a 500-hrs salt fog test (ASTM B-117). Place the scribed test parts in the corrosion chamber and evaluate every 250 hrs. Most powder coatings pass 1,000-hrs salt fog resistance, and 500 hrs is what I consider a minimum benchmark.
Good luck with identifying a finishing technology that provides the necessary cost/performance balance.