Water immersion test for stainless steel passivation

A. Zander,
It really depends on which standard you are looking at, since the wording tends to vary, but I think typically the immersion test is said to use "distilled" water.

My guess is that you'll get very similar results with RO water as you do with distilled, but if you're under a high degree of scrutiny for compliance to the standard, it's usually better to follow the "letter of the law".

Again, depending on the standard, I know there's been some effort lately to genericize the purified water, i.e. ASTM D1193 "reagent water" has moved away from purification METHOD as a classification and more towards quantifiable water quality metrics. So in the future you may find less reliance on "distilled" per se and more a focus on water that is distilled-quality.

A. David:

I hate to present technicalities but in the case of Distilled or Deionized water, they are made by two completely different processes and are technically not the same. For practical purposes they are the same entity.

If the cleanliness of the water is specified correctly you should be able to use either. The best way to avoid problems with your auditor is to measure important attributes of the Distilled water and use DI water that meets those attributes. Keep the data to show your auditor that they are, in fact the same material. If your auditor objects, get him/her to call his/her engineering department to rule on your data.

Some things I have found important in water purity:
1) resistivity (the best, and most expensive, water is around 18 megohms),
2) clarity,
3) silicon/dissolved solids content
4) biological content.

There are others but that's all my simple brain can remember right now.

A. Does your spec state what the quality of the DI water should be? If so, that would be the simplest way, just have a pre-determined conductivity/resistivity #, and once the water quality goes "out of spec", it's time to dump.

DI water will lose its quality very quickly, (especially just sitting in a tank, exposed to atmosphere) even if you are rinsing with DI just prior to putting your parts in the tank. If you really want to minimize the dumping of the tank, a simple, continuous filtering through a unibed should keep your quality in good shape, and minimize dumping.

A. Hi Brigitte,
To establish an interval you could measure the conductivity of the water (ionic compounds dissolved in water). If your water has to be 18 megohm or less, a good in-tank conductivity probe w/ readout would be an option. Hand held meters are available and normally require frequent calibration. The tank would have to be stirred well before a sample is taken, or a reading taken from the in-tank unit. With the tank being unplumbed and aside from ionic contamination, you also run the risk of building up organics, unless you have alternate methods of agitation. Hope this helps.

A. When you refer to rusting in the water immersion process, may I assume you mean the water immersion test after passivating?

Since your refer to different alloys, they must be tested separately. If they are combined, you can get selective galvanic corrosion.

Otherwise, any rusting/pitting is an indication that the surface is not passive, which can be due to several causes. Please let us know what type solutions you are using, as well as analytical controls that you have in place.

In general, corrosion of SS indicates iron particles remain on the surface. So what part of your processing either leaves them or restores them?