Bronze vs Stainless

[ridgy]

When replacing the seacock bolts yesterday, one of them was just a little too long and so couldn't be tightened up enough as the thread stopped about 1 inch down.

Before I realised this I tried too hard to tighten it and the nut got stuck. Some time and scraped knuckles later I got the nut off but damaged the bolt thread in doing so.

The boat is needs to go in on Monday but I can't get another bronze bolt in time. If I used a stainless bolt just for this season then presumably it wouldn't do the bronze much good but the question is, how bad would it be after 6 months?

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[mickshep]

How bad is the thread damage? If the thread was a little too short could you not run a di down the bolt, cleaning up the thread and recutting it a little further down the shaft of the bolt. That way you could still use the bolt you have as the nut would be on the 'new' undamaged thread?

<hr width=100% size=1>My Mum say's I'm not a fat b@st@rd, just heavy boned.

 

[LORDNELSON]

If it is a Blakes sea cock is it possible to place a few bronze washers under the head of the bolt (ie on the outside of the hull) so that you can use a bit of least damaged thread on which to tighten the nut?

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[LORDNELSON]

PS Electrolytic attack of metals can take place very quickly; I forget how far apart bronze and stainless steel are on the noble table but it might be worth checking - have you an anode close by? This might help restrict damage.

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[snowleopard]

if it's Blakes, i think the bolts are imperial threads so you may have difficulty finding one even in stainless.

it is common practice to use stainless ends in a bronze bottlecrew body so it shouldn't be too much of a problem (a bit of grease and an occasional check). I have a similar problem only i sheared the head off mine.

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[Jools_of_Top_Cat]

They also use stainless screws on speed seals for engine water pumps, the pump on my Yanmar is bronze. I queried this on here and Twister Ken wrote back saying he had speed seals and had seen no damage to his pump / screws at all so I guess it depends upon whether your anodes are boding everything and / or you are on a marina or not.

Reading MBM chat being on a marina is the worst thing you can do if you want to retain any metal work on board for more than a season.

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[MainlySteam]

While I walk away from any responsibility /forums/images/icons/smile.gif if the bronze/brass seacock has NOT been attached to or otherwise bonded to a zinc anode ie it has been electrically by itself, and has not shown any signs of galvanic stress eg dezincification under that condition during its life, then you can be pretty confident that it is galvanically stable in saltwater which in turn means means that it should, if neccesary, survive in contact with a 316ss bolt. Such bronzes/brasses seem ok with ss - as an example, galvanically inactive bronzes/brasses used for propellors will live happily together with a ss shaft without anodic protection (and should preferably be without it), again as long as they are electrically insulated from other metals/battery negative in the boat.

If the seacocks are Blakes, then they are likely to be cast from a dezincification resistant brass (before anyone jumps all over me about the brass bit, I suggest that they check their stance with Blakes first) and I suspect that would be fine with a temporary ss bolt. Like any new mix of metals, keep an eye on it for the first few seasons (redundant activity if you replace it next season).

John

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[mickshep]

Re' "Reading MBM chat being on a marina is the worst thing you can do if you want to retain any metal work on board for more than a season." Aggghhhh! Now they bloody tell me. Fosca is all steel below water, Having said that, 1 year on and anodes dissolving nicely, at present rate reckon will have to replace every 2 yrs or so, Mike.


<hr width=100% size=1>My Mum say's I'm not a fat b@st@rd, just heavy boned.

 

[ridgy]

My concern is that it is under the waterline and specifically the bolt head is in contact with the bronze grid on the outside.

316 SS is some distance above bronze, perhaps 304 might be better?

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[Guest]

Unless I misread the question, this "temporary" repair is for a season at least which in terms of electrolysis can be a long time. Last season I saw substantial damage within 3 months on silicone bronze screws holding down a 316 stainless stem band. Mixing of metals is imo unadvisable.

For what its worth, I have always been under the impression that stainless steel, whether 316 or other should not be used below the water anyway as lack of oxygen could cause crevice corrosion.

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[MainlySteam]

Given that silicon bronze is essentially just copper I am not surprised that the ss and fastener relationship did not work. Very dramatic to draw conclusions from that unsurprising experience I would have thought.

Regarding ss under water - what is your propellor shaft and rudder stock made of and do you have some method of keeping them above the water? SS can have problems but again, do not get carried away over it (usually at interfaces with paint or if covered with barnacles or such so is almost totally deprived of oxygen). SS valves are widely used as sea cocks and in salt water circuits drawing (supposedly) unoxygenated water from under the boat - with no special problems that I am aware of. If you are aware of any such difficulties you better tell me quick as I have a big new build for a client being launched on Monday (and another a few weeks after that) which is full of ss saltwater pipework and components.

John

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[Evadne]

Imperial threaded bronze bolts are sold by <A target="_blank" HREF=http://www.classicmarine.co.uk> Classic Marine </A> who I got some from a few weeks ago, they came 2 days later in the post. I think you'll find they are 5/16" by 2". I'd make sure they are machine screws to avoid this problem.
Stainless bolts with salt water in the threads will crevice in 6 months no problem, but, and I'm not recommending this, if you can keep it dry by filling with sikaflex it will not. I replaced 8-9 year old brass bolts this year (yes, I know you shouldn't use them, now). Of the 6 bolts on 2 seacocks 4 were pristine, and two had weakened so the heads snapped off. I reckon it was copious sikaflex that preserved them.

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[Guest]

I bow humbly to your superior knowledge.

By the way, and just in case anyone wishes to have another opinion there is a book "Metal Corrosion in Boats" by Nigel Warren which deals well with this complicated subject

Cheers

Vincent

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[Dirk]

'Given that silicon bronze is essentially just copper'

Nothing wrong with copper. Its more noble than ss. Its the addatives (phosphur, magnesium, tin, zinc etc) that are the the problem.

Any how, there are far too many brasses/bronzes and stainless steels (did you know there are more than 20 types of ss316) to make any ussumption on whether this bolt will cause problems or not.

Im pretty sure the bolt will last the season (vincent has a point, where the bolt is shrounded by the wood and sealant it is likely to corode) but you may find that the bolt has coroded the bolt hole in the seacock. Not satisfactory but I doubt it will make the fitting fall off. Mind you, if your insurance knew they would more than likely have something to say about it.

If you are going to do it anyway, you could do worse than isolating with nylon washers or something.

I believe the new blakes seacocks are phosphur bronze (good quality brass) where as the older ones are nickel alluminium bronze (supposedly the muts nuts).

Liability? me? the only thing ill let you sue me for is poor spelling!

Ta.

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[MainlySteam]

I did not state that I thought copper was a bad material - just that the combination that was mentioned was wrong.

Seems like the galvanic amateurs are at work here. So I am off to my corrosion free boat for the weekend and then overseas to a shipyard where I get paid by professionals to provide the views that some on here treat as rubbish.

Have a nice weekend.

John

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[Dirk]

You are absolutely right. As far as mixing meatals is concerned I am far from qualified. (I normally stick to the 'dont mix dissimilar metals' rule)

I am curious though, copper is more noble than ss right?

If so then surely Vincents stainless band should have gone first. My guess is the bronze bolts had traces of zinc in them.

Dirk

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[ongolo]

Below the water is a lot oxygen, what do you think fish live on?

You can however deprive ss of oxygen when it rests in a cutlass bearing for long time.

Further, most people stll mix up the charachteristics and composition of brass and bronze.

Copper and ss and bronze are very close together on the galvanic scale about 0.05 to 0.1V whereby zinc is almost -0.9V lower.

If you want to know what is compatible and what not, get some sea water insert your materials so that a part remains above the water, then measure the voltage with a digital meter and you will certainly get some surprises. I did it and got some puzzling surprises.

If I (or rather SWMBO) will find my notes, I shall post the details.

One thing however was clear, three different 316 showed different potential against each other. Voltge between my two props and 316 shaft was minimal.

Inserting brass and 316 showed a considerable voltage potential, but inserting some small part of zinc , not electrically connected to any of the brass or ss material, dimished all voltages between dissimilar metals.

The fact that nothing was electrically connected to the zinc, but the zinc afforded protection is against the rule book.

Related to this topic is also the following.

I used to pickle my flat and round bars in a 6m plastic drain pipe of 200m diameter containing hydrochloric acid. One day some body knowcked the bottom out and I could no longer use it. At that time I was almost finished with my hull and did not want to buy another plastic pipe to be redundant within a few days.

I had some 100mm 304 SS pipe and attempted to use the ss pipe for pickling the mill scale off my mild steel. No matter how long I left the mild steel in the acid, the presence of the stainless pipe stopped the pickling process 100%. Interesting is it not?

So I am beginning to believe that we have to learn a few things.

Also using a copper antifouling on a steel hull should cause havoc, but everybody is (or was) using it. Yes I know, the anodes, but how much of the anodes are in fact eaten away by the presence of the copper antifouling and how much of the anodes actually protect the steel?

So that was my contribution to the general confusion.


regards ongolo


<hr width=100% size=1>1000 dream about it, 100 talk about it, 10 start and 1 completes it

 

[cliff]

Electro potential table FYI

Gold Au+ 1.692
Gold Au3+ 1.498
Chlorine 1.35827
Oxygen, Hydrogen (acid) 1.229
Platinum 1.18
Palladium 0.951
Silver 0.7996
Oxygen, Hydrogen (acid) 0.695
Copper 0.521
Oxygen, Water 0.401
Copper 0.3419
Hydrogen (acid) 0
Iron -0.037
Lead -0.1262
Tin -0.1375
Oxygen, Water -0.146
Nickel -0.257
Cobalt -0.28
Cadmium -0.403
Iron -0.447
Chromium -0.744
Zinc -0.7618
Water -0.8277
Chromium -0.913
Manganese -1.185
Titanium -1.37
Titanium -1.63
Aluminum -1.662
Magnesium 2+ -2.372
Magnesium + -2.7
Sodium -2.71
Calcium -2.868
Potassium -2.931
Lithium -3.0401
Calcium -3.8

In theory yes however Cl ions do funny things to stainless steel, Chlorine is found in sea water so there we have a problem. Won't go into the metallurgy of it now but Stainless Steel is not as stainless as people think


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[cliff]

"I used to pickle my flat and round bars in a 6m plastic drain pipe of 200m diameter containing hydrochloric acid. One day some body knowcked the bottom out and I could no longer use it. At that time I was almost finished with my hull and did not want to buy another plastic pipe to be redundant within a few days.

I had some 100mm 304 SS pipe and attempted to use the ss pipe for pickling the mill scale off my mild steel. No matter how long I left the mild steel in the acid, the presence of the stainless pipe stopped the pickling process 100%. Interesting is it not?"

Not really - quite expected. The HCl will have destroyed the passivation of the stainless steel leaving it more electro negative than the iron hence the stainless would have been corroding and protecting the iron.

people ask me 'why did you become a metallurgist', simple - it seemed a good idea at the time!

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[ongolo]

Cliff, there is a little omission in your table. It does not show what metal has been used as a standard reference voltage.

Further even if ss is not passivated it is always more positive than iron.

And here we use a mixture of HCL and peroxide to passivate when nothing else is on hand.

))

regards ongolo


<hr width=100% size=1>1000 dream about it, 100 talk about it, 10 start and 1 completes it