Further on crevice corrosion;- expert advice

[Graham_Wright]

For interest (and with his permission) here follows the correspondence on the topic I posted asking about crevice corrosion in 316 SS. (Bit long winded;- sorry together with some messy repetition).

> Tell me what's happened and I'll do my best to find the answer. Some of the
> issues that need to be identified are:
> Grade of ss
> What is the application
> Is the application above or below the waterline or a bit of both
> Did the problem occur above or below the waterline
> What is the ss in direct contact with
> What material is it bolted to
> What material is used for bolts/screws
> Was/is it to be welded
> What is it welded to
> What is the operating temperature if known. Eg, if a ss prop-shaft, is spent
> engine water being used to lubricate the prop-shaft bearing.
> That'll do for a start.

Point by point;-
Nothing has happened (yet) as the boat is not in the water. The interest I have is in the skeg shoe which carries the bottom bearing for the rudder stock. The designer called for a phosphor bronze casting but this is no longer available and I fabricated one out of SS.

The grade is 316L/1.4404 and the assembly looks like a boot. It is welded up from a 20mm thick "sole" with a skirt of 6mm plate formed to conform to the skeg profile.
The welding rods used were Silvercord variously and collectively defined as AWS A5.4-92 E316L-16, ASME SFA-5.4 E316L-16 and BS2926:84 19.12.3LR.

The shoe is (will be) sealed onto the skeg with a sealant to be decided and through bolted with 316L threaded bar. This bar could be extended to capture zinc anodes each side of the skeg (the prop is phosphor bronze).

There will be an electrical connection to the fixing bars down the inside of the skeg but whether this will be bonded to anything has yet to be decided.

The water environment will be salt, UK to start with and Mediterranean hopefully next year. Temperature range 12°C - 22°C?

Thank you for your interest and I look forward to your reply.
Nothing has happened (yet) as the boat is not in the water.  The interest I

have is in the skeg shoe which carries the bottom bearing for the rudder

stock.  The designer called for a phosphor bronze casting but this is no

longer available and I fabricated one out of SS.OK, from this I am going to assume the part is permanently immersed in seawater.



 The grade is 316L/1.4404 and the assembly looks like a boot.  It is welded

up from a 20mm thick "sole" with a skirt of 6mm plate formed to conform to

the skeg profile.

The welding rods used were Silvercord variously and collectively defined as

AWS A5.4-92 E316L-16, ASME SFA-5.4 E316L-16 and BS2926:84 19.12.3LR. OK. Both the welded part and weld metal are 316L which means a low carbon grade. This minimises the formation of chrome carbide (potential corrodant) in the HAZ (heat affected zone) The formation of chrome carbide has the effect of reducing available free chromium. Chromium is the principal corrosion defense of ss. Below 12% chrome, ss corrodes much like regular steel, i.e. not very well. This situation can and does occur in the HAZ weld metal. Effectively, there are two dissimilar metals immersed in an electrolyte. Ideally, to remove this possibility the part should be heat treated by solution annealing and then passivated. The effect of solution annealing is to dissolve and disperse the chrome carbide rather than leave it as a defined layer in the HAZ. After heat treatment surface oxides and surface impurities all of which are potential corrodants should be removed by passivation. Whoever is doing the solution annealing should also be able to passivate.  However, this treatment, while desirable, may not be practical or convenient, and use of 316L minimises risk of chrome carbide formation. If either part or weld metal were regular 316 you have no choice other than to solution anneal and passivate. Make sure that you sight material specs for both. No-one can tell the difference unless the metal is analyzed.



The shoe is (will be) sealed onto the skeg with a sealant to be decided and

through bolted with 316L threaded bar. OK. same metal. This bar could be extended to capture zinc anodes each side of the skeg (the prop is phosphor bronze).  It is always a very good idea to ensure submerged metal parts are electrically connected to a permanently submerged zinc anode. Zinc is an aggressive anode. As long as all parts are immersed in seawater and electrically connected an aggressive zinc anode will assist overcome pitting and scc (crevice) forms of corrosion.  

There will be an electrical connection to the fixing bars down the inside of

the skeg but whether this will be bonded to anything has yet to be decided.



The water environment will be salt, UK to start with and Mediterranean

hopefully next year.  Temperature range 12°C - 22°C? OK fine



Thank you for your interest and I look forward to your reply.

 

[cliff]

And your question is??

BTW, I would seek a second, professional opinion in regard to your query, but that is only my professional opinion and is worth exactly what you paid for it - nothing (I do work pro bono occasionally)
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[Graham_Wright]

The question lies in the post if you read it completely.

This relates to an earlier post that was responded to by a metallurgist. Would that rate as professional?

 

[cliff]

[ QUOTE ]
The question lies in the post if you read it completely.

This relates to an earlier post that was responded to by a metallurgist. Would that rate as professional?

[/ QUOTE ]Depends on his credentials and experience - Being a metallurgist myself I have met many fellow sufferers but I would not expect say an extractive metallurgist to be ofay with crevice corrosion nor say one specialising in light alloy (Al/MgZn) corrosion to be ofay with welding metallurgy or say bearing metallurgy.

Having reread the post I still do not see the question. The poster is merely posting a copy of a reply he received, A reply which IMHO does not address a number of points hence IMHO the original poster should seek a second opinion.

Corrosion of SS is a complex problem particularily when sea water is involved. In many applications the chosen material is not SS but rahter Cunifer or Monel.

However if he is happy with what appears to me to be an incomplete anaylsis of the problem them he is of course free to proceed as he likes at his own risk and possible loss.
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[Graham_Wright]

My original question was an attempt to feed my paranoia!

I wished opinions on whether I should worry about crevice corrosion in a fabricated 316L SS skeg shoe as I could not obtain a phophor bronze one and had to fabricate instead.
If you have any further analysis to add, all the facts I have to hand are in the post and I would welcome any additions.

 

[boatmike]

A bit harsh Cliff! I agree with your sentiment but he has posted this to try and be helpful to others not to get up your nose and he is not a metallurgist himself......
As a retired Marine Engineer I get pretty livid myself when so called "experts" talk utter crap with such conviction on this site and people ignore my advice in the face of it. However, I persevere as there are a lot of people who come back to me privately for more and thank me for sorting them out which I am glad to be able to do. There are a lot of people on this site who know more than I do about some things and I am careful to preface what I say sometimes with "I am not an expert in this particular subject but".........
There have been a number of posts recently on the subject of electrolytic and crevice corrosion.You would know better than I that this is a particularly complex subject especially when both are considered together and/or one confused with the other. I tried to summarise in a recent post and asked if there were any metallurgists out there who could add anything. Didn't see a reply from you at the time so why are you so critical now?
I can't actually see anything wrong with the post but it certainly looks a pretty incomplete answer so I do agree with you in essence if not in tone. My summary would be that on my boat (which I built myself) the bottom rudder bearing supports (being a cat there are two) are a welded construction (by me) from monel plate. I could also have used a duplex stainless or bronze but as the monel was "available" and suitable filler wire was at hand I used that. I would not personally make them out of 316 stainless at all and believe the best material considering that it would not need a seperate insert to act as a bearing would be gunmetal bronze. However as this guy already has 316 parts the best advice would be to protect them with a local anode I would have thought. The detailed discussion regarding crevice corrosion, oxygen depletion (unlikely considering what the part is and where it is on the boat I would have thought) and the relationship between crevice corrosion and overall electrolytic depletion is a bit superfluous.
Do you not agree?

 

[Graham_Wright]

The original query was posted 2/4/05 and includes the offer to tender expert advice together with the credentials behind that offer.
It would be more helpful if any criticism was additive rather than destuctive.
For my part I am grateful to the contributor for adding to my limited knowledge of the subject.

My backstop position is that if corrosion overcomes my SS shoe with the passage of time, I can remove it and, with a little cosmetic attention, use it as a pattern for a phosphor bronze one.
Any failure is unlikely to be catastrophic as the top bearing will no allow the rudder to drop.

 

[cliff]

[ QUOTE ]
A bit harsh Cliff! I agree with your sentiment but he has posted this to try and be helpful to others not to get up your nose and he is not a metallurgist himself......

[/ QUOTE ] Don't think it was/is harsh. When dealing with as critical an item as a rudder foot/skegmount let's be accurate and clear in our advice. As you point out he is not a metallurgist and therefore would not be expected to look at the problem from other view points (material selection for example).

[ QUOTE ]
As a retired Marine Engineer I get pretty livid myself when so called "experts" talk utter crap with such conviction on this site and people ignore my advice in the face of it. However, I persevere as there are a lot of people who come back to me privately for more and thank me for sorting them out which I am glad to be able to do.

[/ QUOTE ] Fully agree with you, indeed you and I have had some discussions (privately) last year on similar metallurgical problems if you remember.

[ QUOTE ]
There are a lot of people on this site who know more than I do about some things and I am careful to preface what I say sometimes with "I am not an expert in this particular subject but".........
There have been a number of posts recently on the subject of electrolytic and crevice corrosion.You would know better than I that this is a particularly complex subject especially when both are considered together and/or one confused with the other.

[/ QUOTE ]Exactly my point and to put it another way "a little knowledge can be a dangerous thing", unfortunately some give erroneous or misleading advice as if it was "gospel" without the rider "IMHO" or "in my experience" or "I'm not an expert in the field but....."

[ QUOTE ]
I tried to summarise in a recent post and asked if there were any metallurgists out there who could add anything. Didn't see a reply from you at the time so why are you so critical now?

[/ QUOTE ]I did not see the previous post hence did not reply at the time, unfortunately I am only semi retired and do from time to time have to "work". This eats into my free (sailing) time and I do not have time to read every post especially if I have to go back several pages/days to find something.

[ QUOTE ]

I can't actually see anything wrong with the post but it certainly looks a pretty incomplete answer so I do agree with you in essence if not in tone. My summary would be that on my boat (which I built myself) the bottom rudder bearing supports (being a cat there are two) are a welded construction (by me) from monel plate. I could also have used a duplex stainless or bronze but as the monel was "available" and suitable filler wire was at hand I used that. I would not personally make them out of 316 stainless at all and believe the best material considering that it would not need a separate insert to act as a bearing would be gunmetal bronze.

[/ QUOTE ]Certainly one of the better choices or Monel or Cunifer or...... Several options exist all "better" than 316L. However as you continue...

[ QUOTE ]
However as this guy already has 316 parts the best advice would be to protect them with a local anode I would have thought.

[/ QUOTE ]Partly correct, well actually nothing wrong with the local anode theory but ask yourself why use an anode?, what is one of the main sources of corrosion? (without blinding everyone including me with science) - Dissimilar metals in an electrolyte! O.K. now think bush/bearing in the shoe - do we want a bronze bush? no better a synthetic (Tufnol?) bush - that should reduce the work any anode has to do. So yes you are correct as far as it went but looking at the problem from a different angle or perspective we can reduce the corrosion potential.

[ QUOTE ]
The detailed discussion regarding crevice corrosion, oxygen depletion (unlikely considering what the part is and where it is on the boat I would have thought) and the relationship between crevice corrosion and overall electrolytic depletion is a bit superfluous.
Do you not agree?

[/ QUOTE ]To a point yes but.......Let's look at a few things in simple terms first.
Fabrication of the shoe: sole plate ¾"~1" thick? with thinner 1/8"~3/16" welded to the sole plate forming the "heel and half leg" of the boot into which the skeg will fit?. The toe of the sole projecting past the rear of the skeg to hold the lower rudder bearing/bush?.

Assuming one has the desired material, in this case 316L, and the correct consumables including grinding wheels, we then look at the process.
Cleanliness: not manufactured in a blacksmith shop type environment! you must be careful to avoid iron or carbon contamination if the pieces.
Cutting: Plasma cutting followed by grinding the cut edge to remove all traces of plasma waste / oxide. The grind stone must be specifically for SS and must never have been used for grinding Carbon/Mild Steel.
Welding: no preheat and keep the inter-pass temperature as low as possible and certainly less than 80°C.
Minimise the heat input to lowest conducive with obtaining full fusion and good bead profile.
Weld "all round".
Use only SS chipping hammer or SS grinding wheel never use any carbon steel tools.
Ensure 100% removal of slag and "glazing" preferably by grit blasting or power brushing using SS wire wheel. Note: any trapped slag or glazing will result in accelerated corrosion.

Any holes needing drilled should be drilled under-size and finally sized using solid carbide reamer or high speed carbide burr to required size. Use of carbon or tool steel cutters may result in carbon pickup and corrosion problems if contamination is not removed during pickling prior to the passivation process.

"Finished" item should be acid cleaned and passivated.

I will not even bother with the formation of Chromium Carbides and Chromium depletion of the grain boundaries during welding as a) the material is the "L" grade and b) you did keep the inter-pass temp and heat input low - didn't you?

As for not bonding the anode(s), I would bond them electrically to the engine block and the thru' hull anode(s) but that is only my opinion based on some horrific failures I have investigated resulting from rogue anodes and stray electrical currents. in fact I would consider doing away with any anodes fixed to the SS shoe altogether.
The above is only my opinion and I would advise anyone to seek another opinion. Anyone acting on the above does so entirely at their own risk.
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[boatmike]

Yeah! OK, I agree with all that. The preferred material for bottom bearings or the like as far as I am concerned and for the benefit of the rest of the forum is ORCOT (I think I have spelt that right, correct me otherwise) Non metallic therefore no electrolytic problems and unlike nylon is not hydroscopic (does not swell in water).
For your benefit Cliff I am retired and unlike you don't have to work hard anymore. Eat your heart out. Just waiting for the warmer weather! My welding skills are not what they used to be but having been brought up in the aircraft industry originally with Q.C. and welding engineers breathing down my neck I endorse your diatribe on procedures and would add that on my own bits I achieved full penetration welds that passed X-Ray tests by the Lloyds approved QC dept of a large local shipbuilding company (that will remain nameless)
Unfortunately thats not a skill available to most forumites (although there must be some coded welders out there with boats) so they have to rely on the skills of others at premium cost and often doubtful quality sadly.....

 

[Graham_Wright]

Well, we don't all live in a perfect world thank goodness.
Thanks for all the advice; too late to change the material, too poor to invest in X-ray examination and too busy to go to all that trouble with perfect processes.
If it falls off, I'll let everyone know.

By the way, just worked out what "ofay" means.

My (very) young daughter spells it "au fait".

Still, we can't all be experts in everything.

Daughter tells me she looked up "Ofay" which the OED defines as "n. US slang offens. a white person (esp. used by blacks).
Could be racist!

 

[boatmike]

Relax Graham! It wont fall off.... In fact it may never suffer at all. I think my advice would still be fit it and keep an eye on it every year to see if it exhibits problems. I will probably get shot down in flames here but the whole issue is probably overkill. Unless you sit in a Marina with lots of electrical activity around it will probably be OK. In fact I can tell you that when Prouts built cats they fabricated all their bottom rudder bearing supports from 316 (I know because I have the drawings!) and to my knowledge most of them have not fallen off! On my boat I chose to use Monel because I could and it's lots better IN THEORY and didn't cost me anything.
If it helps I assumed that Cliff was quoting the words of that well known Irish Metallurgist Paddy O'Fay..... Didnt think he was attempting French.....
Remember that an expert is someone who knows more and more about less and less until eventually he knows everything about nothing at all......
Don't let your daughter become an expert.......!

 

[Graham_Wright]

Thanks for the reassurance!
We should remember that we are building boats not ships and we live in a practical world not a perfect one.

I am interested in "Orcot" (sic). Tufnol I know absorbs moisture because we machine it using coolant and need to machine oversize to take account of the drying shrinkage (that's practical experience not book speak). For the same reason, nylon is banned in our business because the water absorbtion upsets rf performance.
The condensate of advice on immersed bearings from the site and elsewhere seems to be for UHMWPE (Ultra High Molcular Weight PolyEthylene which we irreverently describe as Umpy).
Fairly cheap, machines well and sufficiently hard not to absorb grit as does HDPE.
I will still fit anodes as I have a 316L shaft carrying a phosphor bronze prop where electrolyticals do matter. Whether I connect everything together internally has yet to be decided. Consider the problems of shore supplies particularly in France where, until recently earthing was for wimps and neutral was used instead. (Practical experience again;- we have a place in France). This is maybe acceptable until Kermit crosses over the live and neutral. Now we have a live marina pool if everything is bonded as per the regs.
The experts can work out the consequences of that!

 

[paulrossall]

I have a 316 stainless rudder step

I took it off and found crevice corrosion inside the step and on the 3 "316" bolts holding it on. I had not bed the step in correctly with Sikaflex. The step has been welded and I will fit 2 zinc anodes to it as there was some galvanic corrosion. The stock sits on the step and takes the whole weight of the rudder. I am considering putting a couple of stainless washers on the step and inserting the stock through to aid movement. I did not want to use tufnell etc. because it might insulate the rudder stock from the step. I want to maintain the connection between the rudder stock and the anodes on the rudder step.
Paul

 

[Grumpybear]

I used to have a leisure 23 with a 316SS skeg shoe like yours. It had lasted 20 years, and I replaced it with an identical one only because the bottom of the stock had worn the hole beyond repair. I sold the boat after a further 5 years, and the shoe was fine then.