Welding Tips and Tricks

GIDAY,
I'VE noticed on my 48 year old steel boat a few places where stainless fittings were welded to the deck and the deck has suffered. The deck, infact all the steel is high tensile and as the boat was built by a boilermaker yet this union of stainless and non stainless has the ordinary steel rusting, I'm thinking its an evil union but I do recall the builder telling me that all the stainless was salvaged from milk tankers as the boat was built in Murray Goulburn Dairies factory in western Victoria and those tankers are made of 309 I guess.

NOW THE QUESTION IS: Would proper flash 316 stainless be less likely to upset the mild steel its welded to ? assuming correct technique and heat management are followed. I'm beginning to think stainless steel is the anti christ as it has been offending the rest of the boats complexion.

yachtweld3mm wrote:GIDAY,
The deck, infact all the steel is high tensile

Would proper flash 316 stainless be less likely to upset the mild steel its welded to ?

Now which is it? There is a difference between mild steel and high tensile steel. 2 different metals.

Do you know what filler metal he used? Dissimilar metals are best welded with 307, 309 or 312 filler.
If he used a plain old mild steel filler weld metal dilution will change the corrosion resistance of the base metals.

When you attach a more 'noble' metal to a lesser one the lesser one will corrode faster. This is because of galvanic corrosion which happens when the connection gets wet (salt accelerates it..) and the metals set up a tiny battery in the droplets and start exchanging current and ions. This starts to corrode the least noble metal.

This is done on purpose with the zinc anodes fixed to boat hulls. They dissolve while the steel remains OK.

In any location where stainless is welded (or bolted!) to a plain steel structure you have to make sure the steel around it is coated/painted in some way to stop most of the galvanic reaction.

On bolted/screwed connections often a nylon or other insulating sheet/washer is fitted between the metals to insulate them and stop the current flowing and thus most of the corrosion.

If you think you have it bad.. Try the guys who have to combine aluminium vessels with stainless fitings... Aluminium is waaaay less noble and will corrode massively in contact with stainless especially when exposed to seawater. (carbon fibre can actually be just as bad interestingly enough..)

Bye, Arno.