Both 304 and 316 are of the family of 18-8 austenitic stainless steel. The difference between the two is 316 contain a kiss less chrome and includes on the order of 3% molybdenum. The "L" in 316L of course means the carbon content is toward the lower limit which makes it less susceptible to sensitization.
Passivation will remove the free iron from the exposed surface, thus improving the corrosion resistance, but the layer of protective chrome/nickel is very thin. Any nicks or abrasion will broach the protective layer. Thus, the damaged area will expose the alloy composition of the stainless steel, i.e., iron, chrome, nickel, and in the case of 316, molybdenum. Now you have two dissimilar metals subjected to the salt, air and water. The result is galvanic corrosion at a rate several times greater than if the stainless wasn’t passivated. The point is, the corrosion resistance is improved if you passivate. However, you must handle it with care so the passivated layer isn’t damaged. Simply handling it with grimy work gloves can be sufficient to breach the protective layer.
Passivation is not usually a field operation since it involves the use of strong acids to etch away the free iron.
The filler metal can be either 308 or 316. There is no need to use L grade if either the posts or end pieces are not “L” grade.
If you are doing any field welding, don’t waste money with passivation.
Al
Al-
Thanks again for the comprehensive explanation. Good heads up on handling, we'll proceed with all due caution. Regarding field passivation- I've located a citrus product that is marketed for this purpose, the guy doing the #4 brush finish is very well versed in metallurgy and agrees with you however the project specification calls for passivation after installation. The polisher indicated if the product was applied to the railings on a good, hot summer day and left for several hours before being wiped off it would do the job decently. He also mentioned he has a different product that I don't know the name of that is a clear liquid application that would extend the anti corrosion quality of the welds by a decade or two and is imperceptible. No field welding will be getting done.
If I understand you correctly- passivation will improve corrosion resistance but post passivation scratches will corrode faster than if not passivated. This railing will be exposed to human traffic, people leaning against it, children playing. In this case would you recommend against passivation. Thank you in advance.
The chromium oxide layer should be self healing in an aerobic environment, no? My understanding of passivation was to basically ensure this layer exists at step one. In some applications, there isn't enough free O2 to effectively fix the layer, hence why you may passivate the stainless from time to time. However, a railing in air should have enough loose O2 exposure to heal, i.e. survive scratching.
This is my understanding. Please correct me if I am wrong or misunderstanding.
As far as galvanic corrosion, 304 and 316 can be reviewed on a galvanic table to see how far / close they are. I suspect they are fairly close to each other and, therefore, have little galvanic coupling. You would have to Google around to decide for yourself.
cheers,
Engineer1984,
Interesting perspective, certainly seems to make sense. I'll be asking the metallurgist at the company doing the #4 brush finish.
Thanks for the idea.
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