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Bertram31.com General Bulletin Board
Re: Coast Guard Response to Ethanol Questions
Posted By: Peter In Response To: Re: Coast Guard Response to Ethanol Questions (Jerry Gomber)
Date: Thursday, 6 April 2006, at 12:07 p.m.
The quick answer is Yes the weld surfaces can be passivated.
Passivated just means the surface is cleaned up and the chromium oxide layer is encouraged to form. This could be done by many different methods, and which method is the best for a stainless steel tank should be left to those who manufacture them.In the simplest form, if you think of how a new stainless rail might get a few rust spots on it in the first season, and then you clean them off and they don't re-appear...well that is the same thing as passivating the surface. The rust occured where there was a tiny inclusion of iron on the surface. It oxidized, and then you cleaned it off leaving a spot where a new chromium oxide layer formed, and the rust doesn't re-appear.
The point is you don't want any foreign stuff getting stuck in any pin holes or crevices right after construction before the oxide layer has yet to form.
In the case of a fuel tank, what you want to avoid would be to have oils or other stuff like fluxes left on the surface and not cleaned off. These prevent the O-2 from getting to the surface and there is a potential that the oxide layer can't properly form.
Another big issue in the past would have been foreign contaminates in the molten puddle of the weld that later can make stress concentrations and cause cracks to start. Water gets into the crack and crevice corrosion begins. The crack gets stressed and lengthens, and so on. That is less of a problem nowadays with modern welding equipment and a good, clean, welding environment.
Another potential problem would be incomplete weld penetration where two plates are joined. If the weld doesn't come all the way through there is a potential spot for for crevice corrosion in the crack left between the plates. Again, a good weld tech in a shop with a modern welder shouldn't have a problem with this.
Putting together a large stainless steel assembly with a welder is not a back-yard or even boat-shed project. The welds need to be sheilded in an envelope of inert gas as the joint progresses, or the strength and quality of the weld will be compromised. Any drafts, blowing dust and other crap can disrupt the gas shield and screw up the weld. It can also be a tricky thing to get the gas shield to behave as the weldor would like when welding in "Out of position" situations, such as upside down, or along a vertical seam. What is easy on a flat surface facing down at the weld can be a good trick when standing on you head facing up at the weld! It is a job for practiced experts in a clean, controlled work environment. In that case a stainless weld should be every bit as reliable as a weld in any other material.
I wouldn't expect that just anybody can get great stainless steeel welds all the time any more than just anybody can do a great job of hand painting the name of your boat on your transom. It takes skill and practice as well as the right tools and environment. But there are fellas out there who can do each beautifully.
With all the potential places to go wrong it doesn't surprise me to hear of tales of failed towers and so forth. If these things were assembled in-situ in a boatyard somewhere there is a good chance that the welds never were right to begin with. Incomplete penetration, inclusions, oxidation of the joined surfaces due to shield interruption, incorrect filler material etc....etc..
My experience is as a mechanical engineer having done a lot of work in the high vacuum field. I dealt with a lot of stainless and a lot of it welded. I am not a weldor by trade or hobby, so it is a bit like an architect telling you about the priciples of house painting. I'm sure an experienced weld shop would have a lot to add to my basic explanation, but I hope this at least makes you feel comfortable discussing these issues with a potential fabricator. If he doesn't have answers, politely move on until you find someone who does.
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