galvanic/dissimilar metals corrosion

[pcrussell50]

i read on here shortly after i joined from some guys who said that stainless bolts will dissimilar metals-corrode more aggressively than stainless ones, in the the aluminum of outboards. i had a hard time believing this when i first read it. but i sure had a tough time removing the lower unit of my big twin '59 super seahorse 35hp, rds-21b. thankfully, i didn't snap any bolts off, [whew!!!]. i ran this topic by my extremely engineer-rich sports car racing forum, and they agreed with you guys. they posted a link to a very informative chart of galvanic potentials, which also mentioned that in extreme environments like salt water, stainless and aluminum are simply too far apart, [too much galvanic potential], and that mild or ordinary carbon steels are not.

so, on the surface it looks like i ought to use grade 8's in place of all those stainless screws and bolts that johnson already had. the screw heads are all munged up anyway, so they will need to be replaced. i use this engine in salt water, but i never store it in salt water. i use it and clean and flush with fresh each time. sooo, it is worth relying on the "science" and using carbon steel bolts, with lots of non-copper anti-seize? the only concern i have is that i think johnson used stainless bolts... then again, maybe they didn't and the stainless stuff was put there by the previous owner/maintainer?

so, think i can get away with non-stainless bolts with anti-seize? [as long as i don't keep the boat in salt water?

-peter

 

[jay_merrill]

Re: galvanic/dissimilar metals corrosion

I think what you would find if you replaced all the stainless bolts/screws, is that the bolts/screws themselves would corrode rapidly.

As I understand the issue, what is going on with the stainless/aluminum combination, is that the aluminum corrodes. Apparently it is a better outcome than having a bunch of extremely rusted fasteners in a motor.

No matter what you do, you are going to have some corrosion in an outboard motor that is subjected to a salt environment, but I think is best to trust the expertise of some very capable outboard engineers, who have most assuredly done a lot of testing over the years.

 

[Vic.S]

Re: galvanic/dissimilar metals corrosion

While immersed in water the aluminium will stop plain carbon steel bolts corroding in just the same way as a anode will, although the aluminium will corrode itself (just as an anode does) albeit more slowly than when stainless steel bolts are used.

Once out of the water the steel will rust quickly as there will no longer be any effective protection by the aluminium.

Heavily zinc plated bolts are to some extent the answer, at least for as long as the zinc plating remains.

When fitting stainless fastenings into aluminium a generous coating of barium chromate paste, as used for stainless fittings on aluminium masts etc should help no end but I cannot say I have noticed it in on the bolts of outboard engines. A good coating with a water resistant grease may be the most practical.

 

[jay_merrill]

Re: galvanic/dissimilar metals corrosion

Vic, you seem to be pretty well informed on galvanic corrosion issues - any opinions of thread lubricnts? I have read that some of them claim to be an anti-corrosion agent, as well.

 

[James R]

Re: galvanic/dissimilar metals corrosion

I have had good success with anti sieze and OMC Thread sealant. Out of the two I think the thread sealant is the most effective. Be sure to coat the bolt or stud shanks as well as the threads. Seal any nuts also. Use in salt water demands special attention so any bolt that may be exposed should be treated. I wish the manufacturers would seal or anti sieze everything during build. Would make life a lot easier.

 

[ezeke]

Re: galvanic/dissimilar metals corrosion

I have spent my entire life on salt water and my experience is that something will corrode, so let it be your anodes.

There are at least sixty different stainless steels so picking the correct one for an outboard is, or should be, left to the manufacturer.

 

[Vic.S]

Re: galvanic/dissimilar metals corrosion

any opinions of thread lubricnts?

No sorry. I have been trying to find a statement that one should not use copper based anti seize compounds on aluminium, does not seem right to me, but have found nothing to say you shouldn't. The makers of Copaslip (Molyslip) do make an aluminium based one though and there's one that contains poly butyl cuprysil (PBC) rather than metallic copper. Don't know if those or similar are available in the US.

sixty different stainless steels so picking the correct one for an outboard is, or should be, left to the manufacturer

haven't counted but more than that in total I'd have thought. I'd expect a 316 (A4) grade to be used especially for salt water use. They'll pretty much all have the same effect on aluminium being quite a way from it in the galvanic series.

The trouble with stainless steel and aluminium is that the volume of the aluminium oxide produced is so great compared with the original volume of the aluminium lost that it just locks the threads together.

 

[dingbat]

Re: galvanic/dissimilar metals corrosion

Copper anti-seize a no-no in contact with aluminum. Copper + Aluminum + saltwater = battery

Tef-gel is the recognized standard for dissimilar metal protection.
http://www.tef-gel.com/contain.php?param=tefgel_infor

 

[HighTrim]

Re: galvanic/dissimilar metals corrosion

haven't counted but more than that in total I'd have thought.

There are actually just over 150 different grades of SS, with 15 of that being the most popular. Ezekes point being even more emphasized though.

 

[DJ]

Re: galvanic/dissimilar metals corrosion

I have spent my entire life on salt water and my experience is that something will corrode, so let it be your anodes.

There are at least sixty different stainless steels so picking the correct one for an outboard is, or should be, left to the manufacturer.

I completely agree.

Engine makers use stainless extensively in their so called "Salt Water Series" engines. There must be a reason.

Also, one needs to consider that outboards are NOT 100% aluminum. They are a made of an alloy. Being that they are alloys, I'll bet the spread between those alloys and SS is not that far apart.

Those alloy recipes are closely guarded secrets.

 

[dingbat]

Re: galvanic/dissimilar metals corrosion

Those alloy recipes are closely guarded secrets.

The vast majority of those "closely guarded secrets" are available at your local library.

If you can't find it there we make a hand held XRF unit that would give up the secret formula in less than 15 seconds.

FWIW: most aluminum alloys are 98.5% or better aluminum. The remaining alloying elements are typically much less stable than the aluminum itself.

 

[ezeke]

Re: galvanic/dissimilar metals corrosion

While it is not a direct answer to the original question, the following article may give some guidance in the use of the fasteners that will help to avoid some of the corrosion. http://www.greensladeandcompany.com/pdf/Problem Solving-Stainless Steel Galling Solutions.pdf

 

[wellsc1]

Re: galvanic/dissimilar metals corrosion

I searched for topic on "galvanic series" of metals and found this thread. Great discussion. I use anti-sieze but it's the silver stuff. I also have the copper stuff, but not for aluminum contact as a post pointed out here.

There's another ally to corrosion, and it's not dissimillar metals. It's stress on the same piece of metal. A stress point on a piece of metal will corroded to its other part when there's an electrolytic to conduct the ionic flow. Think about the bent part of a rod, the threads in contact, or even a pressure point against the metal. It all can be the same metal, but will corrode.

Corrosion protection is a science that is both chemical and electrical. Gas pipeline companies are experts at it.

 

[wilde1j]

Re: galvanic/dissimilar metals corrosion

I've used nonhardening Permatex for many years on all gearcase bolts and water pump machine screws, with never a problem. In saltwater, if you don't do something, the aluminum oxide will fuse the fasteners in place an only heat will help then, and maybe won't be successful.