RRH

I have a potential issue with galvanic corrosion that is probably really simple to someone who understands it better than me... any help appreciated.

Leslie

They bolt zinc blocks onto boat's hulls underwater to stop the steel bits corroding, don't know if that might help.

Les

Scoobychick

iTrader: (1)

Anodes They do clever stuff with electrolytes and positive and negative electrons They are basically sacrificial in that they get eaten away and not the boat hull. That's my knowledge exhausted

Davey L

iTrader: (3)

I've done a very boring course on corrosion, still have the book somewhere. Whats the problem exactly?

Matteeboy

RRH - any more details?
Spent three years doing Materials Engineering - most of it was metallurgy.

MJW

Galvanic corrosion ? Presumably you mean some galvanised steel that's started to rot ? It's a sacrificial method of protecting steel, so if the zinc coating is damaged corrosion can spread

Leslie

Spot on-I'm so impressed!

Les

ZEN Performance

You can get a problem when you have aluminium in the mix, as it often becomes the sacfrifical anode, you can get around to a large extent this by insulating the dissimilar metals. SO in the case of stainless steel fittings into an aluminium housing underwater, oring sealed plastic washers can be used to isolate the stainless from the aluminum.

Scoobychick

iTrader: (1)

Don't be I sell boats for a living, I have to know this stuff in case someone asks

RRH

Sorry, been rather busy the last few days.

We make benches, timetable cases and various other related stuff for the passenger transport industry.

One of my customers has some mild steel benches that need 'attention'- they've asked me to strip, refurb and repaint them.

The bottom couple of inches are rusty on pretty much all of them, so we'll cut the rusty bit away and weld new tube in. I could replace it with mild steel tube, but the same will happen. I'm not going to try and do it with galv tube as the lads aren't keen on welding galv, and the result is never very good when welding it.

I considered stainless, which we can weld to cr4 no problem, and which should withstand the station platforms better, but an associate asked me to consider galvanic corrosion.

_Meridian_

iTrader: (1)

Chromate paste used to be the tool of choice when I did aircraft restoration.



M

ScoTTyB

ask this guy

MJW

There may be a reaction between stainless & cold reduced - is there any way you can mechanically fix new stainless legs to the assembly & put an isolator membrane between the metals ? Alternatively weld new mild steel legs & get the whole assembly hot dipped.

Molbyman

iTrader: (2)

I wouldn't recommend using austenitic stainless steel welded onto mild steel. You can see the effect of this in many gents toilets. Nice stainless steel tubes and brackets screwed into the wall using a mild steel screw. The tube and bracket remain corrosion free, the screw corrodes at a very high rate.
All that will happen is that corrosion will take place further up the leg of the bench.
Its all do do with the galvanic series. Google it if your interested - I'm not gong to expalin :-)

The most cost effective method is to replace the corroded steel with mild steel and, as already suggested, have the bench hot dipped galvanised. There is a Co. in Wolverhampton who could prob do this for you if you need help let me know.

Matteeboy

Very basically, it's best to use the same metal. Even though they are both steel, stainless and mild have different "reactivities" and so encourage electron movement.
Anything with unlike metals touching spells doom - like brass rivets for softops in the 70s. Rust death.

Leslie

That explains it, I have got a good old Shetland in GRP of course but with Zinc anodes bolted onto the outboard motor to go fishing with.

Les