Electrolysis - Aluminium and Titanium

[Scarron]

All my deck fittings are aluminium and during my current refurb I am intending to get them polished and re-anodised.

It is very apparent that there has been significant electrolysis between the fittings and the stainless bolts that hold them down, am I correct in assuming this is due to them being dissimilar metals?

I was therefore intending using aluminium fasteners for non load bearing applications [fuel/water fillers, chain pipe etc.] to avoid differential between metals. I was also thinking of using titanium bolts for load bearing items such as cleats and the Sampson post. My understanding is that Titanium is highly corrosion resistant in the marine environment, but what is it like in the vicinity of aluminium? Will this put a stop to the corrosion?

 

[VicS]

Yes Aluminium and stainless steel are what one would all dissimilar metals. They are quite widely separated in the galvanic series.
Titanium is as you say very corrosion resistant but its position in the galvanic series I think makes it a slightly worse choice than stainless steel as a fastener for aluminium.

 

[Scarron]

Ah good, just what I need to know, that's just saved me at least a fiver a bolt!

Plan B is to use some PTFE/Nylon/heatshrink type sleeving around the bolts to try to isolate the two, I'll have a look into that.

Thanks

Paul

 

[VicS]

Barium chromate paste ( Duralac ) not going to solve the problem?

 

[Tranona]

Or Sikaflex, which I assume you are going to use to bed the fittings.

 

[ODL]

Tefgel is another corrosion prohibiting paste.

From memory is it more viscous than Duralac and can be better behaved when applied ambitiously. I have used both with success.

In fact, could I clarify this with others. Does the gel need to be applied only to where the metals are in contact, or should the majority of the thread and underside of the head also be coated?

 

[rogerthebodger]

Some comments base on my boat building.

My Ronstan genoa tracks came with some counter sunk plastic washers to fit between the track and the stainless bolt. I also used Duralac. When I fitted my aluminium toe rails I fitted some fiber washers in the same way as the genoa tracks. I noted my lewmar traveler track did not have plastic washers but was powder coated so maybe lewmar considered that to be OK. I did fit fiber washers to that also.

Vic your chart does not show the difference between active stainless steel and passive stainless steel, any comment.

I also notice the nickel copper alloy (monel) is down at the same end as stainless steel on your chart yet we don't see the electrolysis between monel and aluminium as we do between stainless steel and aluminium. Could it be the chromium oxide on the stainless that is the real cause ?

Comments welcome.

 

[VicS]

The object of a paste such as Tef-Gel is to eliminate the crevice between the two metals that could trap salt water ... the electrolyte when we are considering galvanic corrosion. No electrolyte means no corrosion even if there is electrical contact between the metals.

So the aim is to completely fill any gaps or spaces between the two that could trap water.

From the website:

1. The function of Tef-Gel in eliminating dissimilar metal corrosion is the elimination of electrolytes from entering the interface of the metallic surfaces. Tef-Gel paste contains 40% PTFE powder and 0% volatile solvents, no silicones or petroleum solvents to evaporate, which would leave voids for electrolytes to be drawn into creating a galvanic cell. When both surfaces are coated and mated with Tef-Gel there are no voids for electrolytes (saltwater) to be drawn in by capillary action over extended periods of time.

EXAMPLE:

A. Bolt with nut or lock nut. Bolt into tapped aluminum, steel, stainless steel, inconel.
Use mascara brush, bottle brush, or toothbrush to apply a thin coat of Tef-Gel to both the threaded hole and the bolt threads and tighten. (Torque requirements may have to be altered according to application of thread lubricants).

B. Bedding surfaces: A closely mating surface (10 mils .010 or closer) can be protected from capillary action by applying a thin coat of Tef-Gel with brush, toothbrush, or plastic scraper and assembling the parts and cleaning excess product with mineral spirits or WD-40.

Tef-Gel does not electrically insulate hardware. The PTFE (40%) is ultra fine powder that will extrude out of the contact area and into the voids of the surrounding areas.

Tef-Gel applied between electrical contacts and connectors does not increase the resistance of the connections even at hundreds of amps, maintaining the integrity of EMI shielding.

2. The function of Tef-Gel in eliminating seizing galling and friction welding of stainless steel, inconel, and other nickel alloys.

EXAMPLE:

Tef-Gel applied to both mating surfaces of nuts and bolts or threaded holes and bolts leaves a layer of 40% solid PTFE within the thread interface, which works as friction barrier. Tef-Gel, which contains 0% volatile solvents, will no evaporate, cold flow, or dry out, giving protection many years later when hardware must be dissembled.​

However where fittings are being bolted to the deck etc it is also important that no crevice exists that could trap stagnant seawater in contact with the stainless and give rise to crevice corrosion.

Perhaps in this situation a good sealant such as Sikaflex to eliminate the crevices that could cause galvanic corrosion and those that could cause crevice corrosion is a wise choice.

I cannot help thinking that in simple metal to metal situations , stainless or Monel fastenings into aluminium spars for example, there must be some chemistry involving the chromate that comes into play. Other wise why use it? Barium chromate has very low solubility but even so there must be some chromate ions going into solution. Maybe they "passivate" the aluminium when other wise galvanic effects would cause corrosion.
Don't know. Any Chemists about?

 

[Evadne]

Ah good, just what I need to know, that's just saved me at least a fiver a bolt!

Plan B is to use some PTFE/Nylon/heatshrink type sleeving around the bolts to try to isolate the two, I'll have a look into that.

Thanks

Paul

Plastic insulating washers and heatshrink tubing works well, I've used it underwater on 316 bolts in anodised aluminium. Some thread has to be in contact with the aly, if that is what it screw into, whatever you put on, but a sticky underwater grease on the threads seems to help keep them clean.

 

[rogerthebodger]

Vic I think you are right it cannot just be to fill the gaps as Duralac does contain a solvent as I know to my cost.

Could this be too different approaches to the same problem, in fact going back to the general electrolysis and anodic protection no electrolyte means no corrosion (boat out of water) or no electrical circuit no corrosion.

There must more than one way to solve the problem

Yes I do know a very good Chemist he lives in a very nice village in Kent.

 

[Bosun Higgs]

Barium chromate paste ( Duralac ) not going to solve the problem?

I found Duralac very effective on a Silette outdrive leg I had on my Prout. This leg is ally, the bolts are stainless and the two are dipped into salt water. Never a problem with a seized or corroded bolt.