Electroplating a Steel Narrowboat In Situ to Combat Corrosion

Probably not.

Electroplating requires current to flow from the sacrificial metal to the substrate to be plated. This would require a considerable DC current for something that size. Plating solutions are also considered hazardous materials and would seriously pollute the waterway.

Haha I'm sure a lot of people wish this were possible....but no, not in open water with known methods....You need a hull survey done and possible over-plating...

https://www.whiltonmarina.co.uk/narrowboat-blog/frequently-asked-questions-about-narrow-boat-hulls-answered/

https://www.boatdesign.net/threads/steel-lifespan.34913/

http://www.belzona.com/en/products/1000/1151.aspx

If the boat was still in the water, lots of other natural ions would be attempting to plate. Lyn is right about the plating chemicals and byproducts being toxic, hexavalent chromium is an example.

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Do you regularly clean the surfaces of your sacrificial anodes? The surface should be shiney. If films build up, the protection is diminished.

Do you regularly check that the anodes are well connected? Poor electrical connection diminishes protection.

Is your sacrificial anode optimum for your particular conditions?

Perhaps anodes need to be spaced closer..

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As far as plating the boat, you might look into brush plating. This can be done in-situ, small areas at a time by hand. The area can't be covered in lake water, but being that is a small area maybe there are solutions other than drydock,

What is the cathodic current demand of the hull in the environment in which it resides...?

Do the anodes provide that (sufficient) current?

If it is carbon steel, is it protected (driven to an electronegativity) of -850 mV minimum...? (have you measured it against a proper/certified reference electrode?)

CP only provides protection AT the "bare areas" ("holidays") in a coating. So, the better the coating, the longer the anodes CAN supply sufficient current to drive the structure (hull) to the necessary electronegative potential for adequate protection.

What you are asking (plating) is, as already suggested, really "far-out-there" (beyond a reasonable expectation, for numerous reasons).

At 10:30pm on this iPhone, i defer/refer-to a previous post:

http://cr4.globalspec.com/thread/114337#comment1217897http://cr4.globalspec.com/thread/114337#comment1217897

The link above appears that work, IN SPITE OF 10 tries over 20+ minutes to "make it right"...(!?!) G'nite...

Thanks Tom. I had not seen that before. Excellent!

Thanks TOM. I have read all comments and the SolarEagle link pages. I cannot answer any of your questions and presuming they would be essential for your NACE forum - I declined to join and I passed up the opportunity. Nothwithstanding you spent a lot of time posting to CR4. For that I am grateful.

However, from what has been said, it seems I am backing a non-runner.

Although I have a nagging feeling that there could be a way ahead.

The rust that occurs over time without any knowledge of effort on my part (I am physically and mentally lazy) could be stopped by introducing a reverse voltage/current - maybe in local spots.

Nothing more than connecting a portable battery/anode device the hull and moved along to do a patch at a time using natural canal water without harmful chemicals as the electrolyte.

lease do not spend time on a research for a complex reply - an off-the-top-of-the-head spontaneous reaction as to whether the idea has merit which from previous comments seems no! ...but I had to ask.

"...using natural canal water without harmful chemicals as the electrolyte..."

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...is a definite, 'no', at least if prevention of corrosion is the goal.

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Still, please answer the following questions (if not for this forum, at least to your own satisfaction):

1. Are the sacrificial anodes firmly attached, and do these remain so until replaced?

2. Is a bright shiney surface kept on the anodes? If any films or gunk or other stuff dulls the surface, it must be removed to reveal a shiney surface. ('All that glitters is not gold, but at least the electrons are free'...which is what you need for protection.)

3. Are the anodes of a composition intended for the conditions the boat experiences?

4. Are the anodes placed with sufficient density? Are any used up markedly quicker than the others? Are there areas on the hull between anodes that seem to experience more or more severe corrosion?

Sorry will not happen.

Best bet is sand blast then zinc epoxy followed by an epoxy sealer, asuming it is worth the cost.

You cannot make iron "run uphill" thermodynamically in a water environment, as it will just release hydrogen due to the differences in galvanic potential and over-voltage required.

It just is not done. Buy a new boat, sorry.

^{Responding to #6 & #7 ~~~}

It would be 'sweet' if CR4 had BOTH a "Browse-First" (_{for Answers}) SECTION as well as a MANDATE for doing-so, before posting... ... It wouldn't have to cover EVERY conceivable topic/question ... but, it would reduce a WHOLE lot of "redundant repetition" of "basics inquiries".

Sadly , our schools nowadays are SO ding-busted busy trying to mold children into "global uniformists", they FAIL to get basic principles of math/language/science etc. into their craniums.

MOST high school grads can't even BEGIN to describe the fundamental concepts involved in the corrosion-of, and the Protection-Against-Corrosion of metals.

"Years ago" (when I was teaching commercial divers to inspect and remediate ships, drill rigs, platforms offshore), I made numerous slide presentations (and an .avi animation) to help get the fundamentals drilled-into-them properly.

Herewith, a few :

This ⇑ is WHY carbon steels, left exposed to weather/humidity, WILL rust (all-on-their-own).

An anode (sacrificial) attached to the steel cannot do any good whatsoever until it is placed into an electrolyte-environment...(hence, anodes on your CAR cannot do a dang thing unless you park it in "Lake Michigan"...!)

B U T ~ once said steel IS placed in an electrolyte-environment, numerous things "start to happen".

The WHOLE crux of the concept is to "drive" (polarize) the structure until it is TOTALLY more electronegative than "WHATEVER POINT ON IT" was exhibiting the ("native") most electronegative potential. Once everything IS "uniformly electrically homogenous", there will not BE any innate tendency to WANT to self-corrode...

IF you can access this "old"/"windows" .avi animation, feel free to download it and use it as you please.

Cheers , and good luck with your hull remediation(!)

I saw some advertisements and articles several years ago about aluminum metal spray followed by epoxy coating. It is a process used in drydock and uses either plasma metal spray or oxyacetylene metal spray. Essentially metal powder is fed into the plasma torch and shot onto a steel surface where it forms an epitaxial metallic bond. Basically the entire aluminum surface becomes a sacrificial anode and the epoxy seals over it. Anywhere the epoxy is compromised still provides it's own galvanic protection.

It is my understanding that it is the outside of the hull that H40 wants to treat, "In Situ".

That would tend to throw a "wet" blanket on spraying molten metal underwater.

I agree. If the boat is that corroded away, why bother at all...take it out of harbor, and give a proper sea burial, and make a new reef (remove coolant, oil, and fuel first if you please).

Otherwise, dry-dock is the only remaining option, sorry.

We pretty much agree that in situ isn't going to happen.

Hence the reference to drydock in the post.

Metal spray looked to me to be an interesting process as a long term solution, but possibly not what the OP would be willing to do to get a long term result.

So you did. I missed that part.

If H 40 decides to go with this method, I'd spray stainless steel (actually it's corrosion resistant) directly over the existing mild steel.

Protecting Ships and Trawlers from Corrosion | applications ...

The hitch with stainless is that metal spray is never 100% dense. It always has porosity. Stainless passivates over time and doesn't provide any galvanic protection to speak of as well as creating sites for oxygen cell corrosion and pitting.

The aluminum, or zinc aluminum alloys mentioned in the article you cited provide galvanic protection even if the water penetrates to the base metal. There are few really good solutions when mixing iron or steel with water. I suppose you could move the boat to the Mojave desert, but the sand fishing isn't very good I hear.

You mean to tell me you never caught a ...wait for it...sand bass? ROFLMAO!

"begging pardon", here . . . but, stainless over CS (in a seawater environment) would spell disaster wherever the overlay becomes compromised.

A scrape below the waterline would cause the steel to immediately become anodic and begin severe local-corroding (pitting) in order to 'protect' the stainless... (this effect would be in the same category as the Liberty ships that sank, dockside, while still being outfitted-for-war... due to mill scale).

[...also, the US learned a $erious le$$on about SS hulls in seawater, in the attempt to build a non-magnetic minesweeper, to get around the (effective) Japanese mines in WWII.]

I was a bit taken aback by (the company, in) your link.

Years ago, NACE had arrived at a consensus that zinc ("galvanizing") was UNSUITABLE for ("longevity-of") any type of immersion service. IF zinc is to be used for sacrificial protection, it should be accomplished using properly sized/spaced anodes in conjunction with a suitable coating on the steel. At the same time, it had been agreed that TSA (thermal sprayed aluminum), also with a proper topcoating over itself, WAS an acceptable practice for immersion use (e.g. diving bells).

We see effective protection from coated TSA on deepwater "risers" on numerous structures out-in-the-deep (Gulf of Mexico).

FYI , the reason that galvanizing is not considered suitable for immersion use is that the relatively thin coating does NOT exhibit suitable "throw" (NACE term for how far the polarization can be delivered, across any area that becomes bared, either by damage/wear or by consumption (sacrificial).

I would be interested to see any "newer"/"updated" data that is to the contrary.....

{even this 'old dog' is willing to "re-learn" a thing or two, when (as Katherine Gobel-Johnson said, in that wonderful uplifting movie "Hidden Figures")... "the numbers don't lie".}

[Edit: I see that JP jumped-in while I was typing, and, note that I agree with {his} paragraph #1, and, "expound-upon" the 2nd paragraph...]

The only reason I could see zinc metal spray being acceptable is that the spray process typically applies a thicker coating than dip or electroplate galvanizing. The aluminum spray I had seen before was 1/16" or thicker and actually had some substance to it. I would agree that you wouldn't want to run it bare in any case and that a good topcoat would be absolutely indicated. I could see a legitimate concern that adding the thickness of metal would significantly kick up the hull weight.

re: "...the thickness of metal would significantly kick up the hull weight"

Hmmmm...

"First prompt" , here , to interrogate the differences-in-performance that are to be expected between three different applications of zinc ("_{typically-applied-versions}"):

[electroplated vs. HDG vs. thermal-spray applied...]

Electroplated galvanized steels have the pure zinc outer layer but lack the intermediate alloy layer that mixes the steel and zinc.

versus: Hot-dip galvanized (HDG) steel ... their appearances are different

...(though sometimes difficult to recognize the difference after a period of weathering...)

( 2nd image from here ... lost the link to 1st one )

BOTH of these images depict the 4 differing-layers that are formed by the HDG process.

Needless-to-say, thermal sprayed Zn would exhibit a whole different cross-section, and thus, SOME difference in (cathodic protection) performance. The weight of any sacrificial-metal-coating will certainly affect its longevity (as well as "up front performance"). I'll admit to NOT having a comparison between "them" (any)...

Yet , I'll still bet dollars-to-donuts that a proper protective coating (engineered for the task) with properly spec'd anodes would be considered superior, in a brackish/seawater environment, over a "zinc only" application.

The above, IN SPITE OF the "world-record-performance" of the IOZ coating applied to a ("_famous") >50-yr-old pipeline in Australia. (That is an "over-land" / above ground pipeline, and "inorganic zinc" coating ... so, not an "apples-to-apples" comparison)

In light of most all the comments being relative to aluminum and/or zinc thermal spray, I am actually quite surprised that no one has mentioned the use of water-proofing epoxy coatings such as Belzona, or other proprietary formulations that work extremely well in brackish water corrosion protection of steel.

In our circulating water lines at work, we have such a coating applied after sand-blast cleaning, and I typically see iron residual concentration in the circulating water (from cooling tower) at < 10 ppb. I never saw such low iron values on un-coated pipe. Usually this was at least 10 times higher.

"surprised that no one has mentioned the use of water-proofing epoxy coatings such as Belzona"

In the original post, he "...wondered if the electrolytic erosion process that accelerates the rusting can be reverse engineered with the boat still in the water to create an electroplating system to fill the pock marks"

He admitted to "painting with bitumen"... and also, to being "physically and mentally lazy"(^_#7)...(^{_{aren't we all, at times}}...).

In the middle of my post#18, I'd reiterated (^{_{from training}}):

"IF zinc is to be used for sacrificial protection, it should be accomplished using properly sized/spaced anodes in conjunction with a suitable coating on the steel."

Certainly, Belzona is one of many companies involved in engineering GOOD solutions to a variety of protective coating needs.

Around my-neck-of-the-woods, International Paints gets the 'lions-share' (^{_{from what *I* have seen}}) of work, protecting our offshore oil-&-gas works.

They even cover the occasional "big-time-outsider", now and again:

<< linked

...... but , I don't get the impression he is anxious to lay-out the necessary funds for EVEN the scraping/blasting away of his 'bitumen' ... let-alone the dry-dock 'painting' of his "pride-and-joy" with a high-tech protective coating solution.

"PS" : ("wait-for-it" ... ^{_{NACE joke coming...}})

paints are for "aesthetics" , protective coatings are for protecting investments.

? What's the difference...?

"About forty bucks a gallon" . . .

sounds about like a hen way to me, or the difference between a duck.

Sorry, but my mind being stood on head by comments in the earth orbit argument...

I have decided to just run up the white flag, and go with the flow for now.

"hen way to me" ... ehhhh?(^_???) ^{_{hen way , hen weigh}}

"Sorry, but my mind being stood on head by comments in the earth orbit argument"

You totally LOST me there.... ((where's the argument? I might wanna chime-in))

L-I-N-K-S ... !

effect of lunar rotation etc

I still think velocity has to increase with orbital radius, but apparently I am dead wrong.

I have already had the "whip" cracked on my knuckles during that conversation.

Hello ndt-tom. You have spent far too much time on this subject on my account - but thanks anyway. As have many others. My anodes have nearly gone, which doesn't help.

Typically and traditionally a boat is taken out of the water, cleaned, and painted with bitumen. but probably now with 2-pack epoxy.

I am trying to decide what to do about the rust. In my case I am faced with giving up my boat - mainly because gradually failing physical abilities of my self and my wife due to advancing years means we find it hard to handle our boat and locks. Sad really.

The value of the boat will depend a great deal of the condition of the hull - and therefore what to do about. It is not a DIY job for me anymore.

Enquiries at boat yards produce the same sort of solution and cost. Cleaning and blacking as basic, or shot-blasting and 2 pack epoxy, or over-plating if rusting is severe. Apart from the method of treatent - it is the logistics of getting the boat to the yard and dry-docked, inspected (independently, after cleaning, and again after painting - and finally valued.

...and whether the extra cost and hassle is worth it, or sell at a lower price.

None will commit themselves to the increase life of the boat for each method.

Comments above about the 'paint' are interesting and worth exploring. Thanks for the info.

However, it surely comes down to the effectiveness of the cleaning process - a wire brush process hand is easily skipped over and covered up with paint, and so could shot-blasting I guess.

"it surely comes down to the effectiveness of the cleaning process"

MOST assuredly.....(!) It was the US Navy, in those "war years" ("40's") that studied and ultimately taught us that ANY coating being applied to a seagoing vessel will not stand the proverbial "chance in Hades" of staying where it's put without having a sufficient "anchor profile", to which it can cling.

Proper blasting became the "norm" (along with a plethora of "cleanliness tests"; testing the blast media itself, the blasting air {for oils, other contaminants}, and the surface afterwards, BEFORE applying the coating.

NACE (and SSPC, which used to stand for Steel Structures Painting Council, but now without modifying the acronym, stands for Society for Protective Coatings) maintains the Standards... many of which are referenced in the US Code of Federal Regulations. {our government, while being quite foolish and all-too-"spend-not-so-thriftily", DOES take our infrastructure seriously, and, in order to allow-for the tax-breaks to industries such as the energy sector, mandates that they maintain that infrastructure appropriately..."for National Security's Sake"}

Anyway ~ I sympathize with your "plight"... my father lived his first few years of retirement on a boat (a "live-aboard-trawler") in a marina in San Diego. THAT boat was sold for "a bit less than was hoped for"... and, a few years later, my own boat (in Florida) was sold for a fraction of what I'd hoped for. Working 60-70 hours a week didn't leave enough time for proper maintenance, and... well, it needed "bottom-end" (drive) work, as well as exhaust manifold, heat exchanger...(and trailer work to boot!)

I wish you well --- advertise the heck out of it, and wait patiently. (Have a "party" on it now and again!) --- "somebody" will come along eventually, and 'bite'.

Thanks ndt-tom for your technical help and comforting thoughts. The battery negative terminal inside the boat is bolted to the steel hull (earth) - although earth return is not used as a conductor in any circuit. All devices have cables for neutral return.

But one point to clarify please, and I shall get some stick over this for asking:

I should be able to work it out for myself.

I am confused about the polarity. By convention current flows from positive to negative - whereas electrons flow from negative to positive - so is the lump of zinc/magnesium bolted to the hull - which is negative according to the battery, a cathode. but it is called an anode. Is it at a negative or positive potential.

Put another way, in terms of simple wiring, if I connected one end of a wire to the battery positive in the boat and the other end to a lump of galvanized metal, and dangled this in the canal close to the boat - would the zinc transfer to the steel -

Or vice versa, because I can see how the boat electrics inside on the dry side of the steel are totally separate from the wet side. So I could use a totally separate battery and connect the positive to the hull, and negative to the zinc.

What would happen to the zinc/steel ?

Never ever set up the steel as the anode in a circuit. The zinc (if you imagine a cell made from a zinc electrode and a steel electrode with electrolyte between them and an external connection (wire)) will be the galvanic anode, and will send electrons into the external circuit. The steel is more cathodic than zinc, and will be receiving electrons from the external connection (wire), although the normal way to protect the steel is a direct, low resistance bond with the anode zinc (or aluminum or magnesium).

Since the sacrificial anode material is connected the potential on the steel moves to a protected level with respect to the iron corrosion reaction with water (almost as if the pH were raised to a protected value above 9).

The steel hull should be grounded in accordance with the rules about marine electrical service, and in general does not shift the galvanic potential of the steel away from a cathodic protection level. Granted that one never sets up the hull to be a bus bar (i.e. a current carrying conductor).

Thanks James. I like the analogy to a battery. I can understand a zinc electrode in water (with some acid ??) in a steel cup producing a voltage between the electrode and cup, and electrons flowing from the steel cathode to the zinc anode through an external wire connected between to two.

But what happening to the 'metal' - atoms ? molecules ? on the electrodes - are they transferring ? - or will that only happen if an external voltage is applied to reverse the flow of electrons.

To the extent that current flows in the external circuit, the zinc is oxidized, while nothing happens to the steel, other than perhaps some reduction of oxygen-containing compounds to lower oxidation state, or release of oxygen, depending on the over voltage involved.

If one were simply testing this circuit with a high impedance input (to measure the cell potential, for example), the zinc would not corrode away any time soon, at least not in quite brackish water.

Again, only have another minute here, but..... RE: "electrons flowing from the steel cathode to the zinc anode through an external wire connected between to two"

"Measure THRICE, cut once" ... ("think twice, state-it-right")...

I hope this image comes out readable (darned editor likes to muss-up graphics!).

Just in case it doesn't ; electrons cannot and will not flow freely through (an) electrolyte. They ride "piggy-back" on ions through the electrolyte, but flow freely and easily through the metallic path, FROM the anode TO the cathode.

And, anions (in the electrolyte) gravitate towards the anode, while cations (you guessed it) gravitate towards the cathode.

ndt-tom. Thanks again. It is not fair to keep putting basic questions to you and the forum that seem to arise from a very rusty memory of current flow convention.

It would appear my idea is not practical or viable, if at all workable.

But to be clear, my boat is not a rust-bucket. It is in good condition for 30 years old, albeit rusty externally below the water line, but perfectly dry inside with absolutely no water leaks anywhere (somewhat unusual for a narrowboat considering the plethora of bilge pumps on the market) - so no rusting there.

I will abandon the idea - and look at the 'clean-and-black' process that everyone uses - perhaps sell at a discount and leave it to the new owner to decide.

And as an off topic digression - what is the principal factor learnt over the years by engineers - do you maximise the value of the old asset by spending time and money on it - or get rid of it quickly and cheaply as possible - and devote your efforts and money to the replacement.

Thanks Tom. In my case demands on time will probably force the issue. Our boat is moored at the end of our garden at the moment, and working on it has been a hobby for years - with occasional cruising - but as we are likely to be selling our house soon, and looking for a smaller place (it's an age thing!) and coupled with the fact that we are finding that trying to handle a 15 ton boat when we haven't the strength and agility anymore, puts us at some risk of having a serious accident. So reluctantly, we are faced with selling it when we move. In all probability we will not have the time to work on our boat or do anything more than token cleaning - so as you say the decision is a can of worms governed by many factors - the governing ones likely to be a time limit with a small window of opportunity dictated by external cost and time pressures of the house sale not related to the boat itself. On balance I tend to find favor with the 'easiest' way out to avoid hassle with after-sales problems. But in response to Andy's post I have sent and email to Evoqua to ask about their ICCP system.

Perhaps consider trading down, maybe by building yourself a boat with requirements that can be met easily just (literally) single-handed.

There are some nice skin on frame examples out there....

It was a nice thought, really nice, but I REALLY doubt that a Canal Boat owner will go in the direction you posted....

Even "small" canal boats, if they exist, will still be a problem (guessing only!)......

I took to the road after living on water, by buying a caravan (now on my 3rd!)......I still feel like a captain, of my "Ship of the road"....

I travel faster quicker and stay longer!!!

ICCP systems DO offer some advantages. They can protect vast expanses of bare steel, even in severe marine environments.

BUT : do be aware that they afford absolutely zero protection when the power goes out. ((and.... there is absolutely zero, zilch, nada in the way of "electroplating/repair" of a surface when using such a system. All it can do is afford the polarization protection.)) There are a considerable number of options, for the active anodes. Some manufacturers push "investment metals" (think, "gold plated") as they show little-to-no appreciable wear after many years of service.

Wishing you well... ((I, too, am at the age of not wanting so much maintenance on my back, and as such, I am looking forward to selling my residence... in the not too distant future.))

(^{_{Sorry, only have a quick sec here, but, looks like James "beat-me-to-it" anyway}})

I'm unsure whether you are considering an "electroplating" sort of schema, or an "ICCP" (Impressed Current Cathodic Protection) setup... but.... impressed current setups require that the ANODES (hanging in the electrolyte) go to the positive terminal, as we always want the protected structure to be driven electronegative (see my post above, #9).

That is, UNLESS we are establishing ANODIC Protection, instead of CATHODIC Protection.

Anodic protection is what gets applied to "SS" sorts of vessels, wherein we need to drive things in reverse in order to ENHANCE that passive layer on the SS.

That is the only scenario where "anodes-go-negative" (from a "hook-up" viewpoint).

Will check back at end-of-day if I can....

Strangely enough, I saw one vendor page where they explicitly stated impressed potential protection for sulfuric acid service in carbon steel vessels, where the acid was not 98% sulfuric acid (but lower, say 66 °Be, 93%), where the steel was driven anodic for some strange reason. I seem to recall this having to do with producing a uniform small loss of material at the get-go, but quickly being passive by the formation of a tight layer of ferrous acid sulfate that did not continue growing in thickness, thus the current draw dropped off sharply.

"Ahhhhh" ... you most probably are remembering correctly(!) Chemistry was never my "strong suit" (though I've been blessed with some understanding, more than 'many').

I do vaguely recall some "peculiarities" with respect to the different requirements for production and storage of various acids ... including that oddity regarding nitric acid... [the potentiodynamic polarization of stainless in nitric acid service, and also the transpassive breakdown potential (in storage) being reached AS the acid becomes more and more diluted]. I can't recall whether it was a (^_ugh) "Bill Nye" or a predecessor's show, on which I first saw {as a teenager}, the SS coupon in a beaker of HNO₃, wherein the SS was "quite happy" until (^{_{pure / drinkable}}) water gets added. That flashback came back to me when seeing it repeated in a NACE classroom(()).

It was also in one of those classrooms wherein I heard for the first time how much sulfuric acid is actually produced, globally... and, that in many such applications, carbon steel is the 'transporter-of-choice', simply allowing for the (well-known' corrosion rate, and replacing as necessary.

Still ... to the "OP" ... (as you've already stated) "plating-in-situ" ain't gonna happen.

In simple terms, I am looking for an easy way to protect rusty steel of a narrowboat in fresh water from further rusting. An in-situ electroplating process instead of lifting out and painting.

It will never work. Plain and simple.

I don't think you should attempt loading extra jettisonable ballast and running aground in shallows at high tide and brush plating various sections at successive low tides.

This is not the first time I have heard of the term "throw" in reference to cathodic protection. It comes up all the time in considering condenser anodes, how many, what kind, etc. Magnesium and seawater is a bad idea. That is why aluminum is the choice in marine environment.

There are also the consideration of throw in regards to installation of permanent impressed potential cathodic protection systems in water pipelines, for example.

The calculations are really more art than math.

"Magnesium and seawater is a bad idea" ... except that for the initial "splash" (introduction into the environment) some "big-dollar" installations will have a layer of magnesium applied on one face of their Zn-In-Al anodes, so as to afford a "lickety-split" polarization of the structure, in order to insure that NO toe-of-weld corrosion will be experienced.

"That is why aluminum is the choice in marine environment" ... Zn-Al, with an "activator" to KEEP them active (in spite of the natural passivation that occurs to either pure zinc OR pure aluminum, in the carbonate-rich seawater). Decades ago, mercury was utilized for that purpose {particularly in anodes that would be subject to getting "buried" by the mud on bottom}... but, EPA pressures, I am sure, contributed to Indium gaining higher favor.

"...also the consideration of throw in regards to installation of permanent impressed potential cathodic protection systems in water pipelines" ... "aye that!"

Water pipelines (AWWA 'jurisdiction', as opposed to API {oil, gas, and hazardous liquids}) learned their OWN "corrosion-specific-issues" over the years...(!)

It was in San Francisco, after the first (electric) trolleys were installed, that we (as a country, so-to-speak) first learned the peculiar HAZARDS from stray currents...

Corrosion protection of the interiors is a whole-'nuther-art (as you suggested), from protecting the exteriors...

Yes, many engineers major in protection of one side of the pipe or the other. Mainly us water-treaters focus in interior pipe protection, while totally forgetting about the environmental effects outside the pipe that slowly but surely compromise the integrity of an entire system or subsystem.

".....also, the US learned a $erious le$$on about SS hulls in seawater, in the attempt to build a non-magnetic minesweeper, to get around the (effective) Japanese mines in WWII..."

.

This is interesting to me. I did attempt several searches for US WWII stainless steel hull minesweepers, but to no avail. Have any links handy?

That was one of the lessons shared in one of the (many) NACE courses with which I was blessed many-years-ago.

The military(/government) poured buckets-&-buckets of money into a minesweeper, the focus of which was to insure "non-magnetic" characteristics throughout its hull.

Sadly, it had to 'limp-back-into-port' before finishing its maiden sea trials, as it suffered welds popping 'port-and-starboard' (^{in lieu of 'left-&-right'}); and, we subsequently learned the dangers/possibility of CSCC (^{Chloride Stress Corrosion Cracking}) of stainless steels. Fortunately, thru the years, many companies and institutions (who do "real" science) have formulated a veritable plethora of new stainless steels and alloys that provide the PERFORMANCE desired (strength with corrosion-resistance) while providing those performance capabilities IN a multitude of different corrosive environments.

Many people ("laypersons") fail to realize (to "make the connection") that the mechanism of protection is not afforded to the steel (SS) "directly" by the inclusion of certain constituents WITHIN the alloy itself, "per se"... but, that it is the unique characteristics that are imparted-TO the micro-thin "passive layer" that develops on the surface of stainless steels, bonded SO tenaciously as to equate to a Star-Trekkian "invisible force field"... and that passive layer is different on each alloy, due to the characteristics imparted to it BY the various alloying constituents.

If ONLY they had known all these things back in the "40's........

A later approach of the navy was to inject a standard hull full of Styrofoam. Didn't matter if it set off a mine as there was nowhere for the water to go if it got through the hull.

I suppose an approach to consider is injecting the boat with Styrofoam, let the steel rust away and fiberglass that foam core.

BTW on the aluminum metal spray, there was no intermediate phase at the aluminum/steel junction and no mixing of metals. The joint consisted of epitaxial growth of crystals across the material boundary. The aluminum did not stay liquid long enough after contact to diffuse iron into the aluminum droplets.

"...inject a standard hull full of Styrofoam"

...wOw... THAT had to make things difficult, getting to the "rudder-room".....

"^{_{no intermediate phase at the aluminum/steel junction and no mixing of metals}}"...

"Aye" that ... much like the TSA used routinely now, on many marine structures (as previously mentioned in this thread).... as on this "coupon" in my office here:

Don't know where you're at, but... if you're ever in San Diego (as I was many years ago, at a NACE conference), a visit to Flame Spray Incorporated is a real "eye-opener"...(!)

THOSE fellows know the business, and zinc / TSA are "minor-league-stuff" compared to most of the exotic coatings that they apply, there.

I was truly astonished at the levels-of-control that can be effected, using different groups of materials and different methods of application. Truly a science-unto-itself!

Give these fellows a call. At least you are in country with them.

I bet they might have some wise words of advise...I ask, is your life or you loved one's lives worth a ride in that old dilapidated shell?

It does seem that the possibility of somebody engineering a biological coating that could restore the finish might be somewhere just over the horizon....

http://spectrum.ieee.org/nanoclast/semiconductors/materials/cyborg-bacteria-change-the-game-in-carbon-dioxide-reduction

I would not count on it happening in a timely manner....

Or the side effects of releasing such a bacteria into the general biosphere.

Would it cause grandmaw to be un-run over by a reindeer? Or would it cause all drunks to be released from prison, and all cowboys to get a new pickup truck?

Please let's not play that C&W record backward, it might cause all manner of havoc in the economic system.

Hey, another important aspect of this problem is of what material the hull is comprised.

If this is a mild steel and pitting corrosion is occurring, since pH is presumably not extremely alkaline (guessing it is under 9), then there are probably other significant factors to this pitting corrosion.

Is the boat kept in a low O2, low flow area? Is there significant sulfur in the water, or have you noticed a sulfur smell? Any notable industrial or other effluent near the place where the boat is kept that might lead water to be unusual?

.

Any chance you could post some pictures of the corrosion?

Mild steel

No pics - it's all underwater.

Normal UK canal water - clean(ish) supports fish and wild life. There's even fresh water mussels in the muddy canal bed.

Not quite what you asked, but the Royal Navy, at least when I serve, had special equipment to nullify those currents. Its a bit similar to electroplating, nullified.

Surely there is something better is on the market today?

I found this for larger ships:-

http://www.evoqua.com/en/brands/Electrocatalytic/Pages/what-is-cathodic-protection.aspx?stc=ppc300054&ads_cmpid=165498671&ads_adid=11198644751&ads_matchtype=b&ads_network=g&ads_creative=166041662769&utm_term=%2Bship%20%2Bcorrosion&ads_targetid=kwd-69367608644&utm_campaign=&utm_source=adwords&utm_medium=ppc&ttv=2&gclid=CjsKDwjwuvjNBRDPhu6d55nyTxIkAKWKtM4s6MErSR20dUCoW43xb4xlABWYDiMn279EvdAn93tQGgIhofD_BwE

I did see (there is a Guy over the road who repairs and sells sporting boats) that some people "paint" the cathodic protection bars.....keeps them pristine of course!!

Maybe you simply need more bars in more places and make sure that they are fully and correctly installed AND NOT PAINTED.....

They must have good electrical contact with the hull of course.

Other than that, I have forgotten how such things work.....sorry.

But I would have thought that there are better coatings around nowadays. In the direction of fiberglass reins, but even better...?

The pin holes might improve adhesion?