Salt water corrosion on cast iron keel 3

[berkshire]

I am responsible for the upkeep of 10 keelboats with cast iron keels.
I am being bugged by localised failures of the paint systems. Three years ago I had the boats hauled, the keels were sand blasted to grey metal,then painted with clear penetrating epoxy primer,followed by epoxy mid coat then antifouling for top coat. There are no anodes on these boats.
At random locations on the Keels, a blister will appear, split the paint film and the iron will start to rust.
I am about to put three boats into the yard on Monday for renewal of the antifouling.
If I leave this up to the shipyard they will scrub out the blister, spray a bit of zinc chromate primer in the blistered area and slap on the antifouling.
This to me does not seem to be an adequate fix.They tell me thats the best they can do, because they are no longer allowed to sandblast.
Anybody got any better suggestions? This area is very tight on VOC's.
B.E.

 

[MikeHalloran]

Duro/Loctite "Extend" might be worth a try as a primer. It doesn't stick to bare metal very well, but when it's allowed to convert iron oxide to whatever it converts it to, it seems to work pretty well. So just remove whatever loose stuff you can, and leave the adherent rust. It's applied as a latex, so VOCs shouldn't be a problem.





Mike Halloran
Pembroke Pines, FL, USA

 

[EdStainless]

Clean surfaces with abrasive flap wheels or spring wheels. This should let you get back to well bonded areas.
A base coat with a conversion primer is a great idea. These often need extended cure times before top coating.

= = = = = = = = = = = = = = = = = = = =
Corrosion, every where, all the time.
Manage it or it will manage you.

http://www.trent-tube.com/contact/Tech_Assist.cfm

 

[berkshire]

Mike,
Is the Duro/Locktite "EXTEND" a phosphoric acid conversion coating?
B.E.

 

[MikeHalloran]

It's _some_ kind of acid; it will stain your fingers, and I'd describe the smell, while subtle, as acidic.



Mike Halloran
Pembroke Pines, FL, USA

 

[berkshire]

I understand that some epoxy coatings do not, like acid conversion coatings because the acid reacts with the amine hardeners in the epoxy coating.
B.E.

 

[MikeHalloran]

So, use a barrier coat of ... anything.




Mike Halloran
Pembroke Pines, FL, USA

 

[berkshire]

The original coating was made by Ameron and was their preprime 167 followed by their mid coat.I guess I should have asked them why this system failed before asking here.
I will look into seeing if I can get an iron phosphate coating in place and excess acid neutralised before the yard does their painting.
Thanks Mike.
Brian Evans.

 

[berkshire]

EdStainless (Materials)
I like your idea best, one thing I have noticed about these blisters is that when they are removed the rusting into the iron keel is severe, sometimes 1/2 thick. I have used a needle gun on some of these places and find pockets into the iron the size of your thumb nail. Could some of this be electrolytic corrosion? It seems to be a little rapid for general oxidisation.
B.E.

 

[mshimko]

Ameron's Pre-Prime 167 seems to often be in the mix when discussion premature coating failure.

I used it with Ameron's recommendation over 10 years ago in a few trial applications; I found in less than a year their "standard" epoxy worked better without pre-prime.

 

[berkshire]

mshimko (Materials)
Thank you. This is good to know.
B.E.

 

[ssreese1500]

Berkshire,

Is the pitting only happening in the keel or is it happening in the rest of the underwater surface of the vessel??

What you are describing sounds like you might have a ground in your wiring going to the vessel hull. Also, are you using zinc or aluminum anodes to provide any cathodic protection on your vessel? If you do, and they are gone already then I would say you need to investigate this as your main problem. Another indication of this is if the seam welds on the hull are wasting away badly.

A lot of yards are getting away from grit blasting and are using HP Water Blasting instead. The water will take the paint off, but it wont "feather" the edges of the paint like grit blasting will for spot coating. Here is another thing, when you water blast the steel flashes to a surface rust coat immediately. The most important thing to know is this: Zinc Coatings are not very surface tolerant, and do not work well with water blasted surfaces.

You want to fix this fast and have it hold up for 5 years, heres what you do. Put the boat on dock and pressure wash the entire area you need to repair with a 3000psi pressure washer. Then mechanically clean the areas where the bare steel is exposed, or if you have paint bubbles you want to repair. Make sure that the paint on the hull has good adhesion to the hull (a pocket knife blade should have to be pushed really hard to get under it) After this is done, spot coat the areas of exposed steel with Ameron 236 two part epoxy coating, making sure to have a good overlap onto the coating still on the hull. After 10 hours, but before 24 hours is up, put on another spot coat of 236. 236 is extremely surface tolerant, so dont worry about if there is a coat of surface rust on the steel.

Lastly, touch up your anti-foulant. Do this right after 6-8 hours if you can, as AF should be applied to epoxy while it is still tacky.

Hope this helps.

 

[berkshire]

ssreese1500 (Marine/Ocean)
Thank you for your response. The three boats are now back in the water.
The yard ground out the affected areas with a disc grinder, the reprimed the affected areas with Proline paint's epoxy chromate primer followed by their epoxy mid coat,followed by antifouling. The original coatings were Amerons system preprime 167 then 236 then Amerons ablative antifouling. We are now no longer allowed to use the ablative antifouling due to tightening of regulations regarding free copper.
We did find black areas under the blisters where the cast iron looked almost like graphite. I am now investigating stray electric currents from other boats in the marina where the boats are docked.
B.E.

 

[mshimko]

For ssreese: This is off-the-original-topic, but I've a question on your last post. You mentioned "We are now no longer allowed to use the ablative antifouling due to tightening of regulations regarding free copper"

Can you elaborate? I know there is a trend towards tightening reg's on copper containing paints, but I wasn't aware of any actual restrictions in place at this time. Were there actual restrictions, or is the marina restricted, or are one of you merely thinking ahead?
//mjs

 

[ssreese1500]

Mshinko, first of all that was berkshires post, and I am a little familiar with this, but am certainly not a expert, but I will tell you what I do know, or at least what I have been told.

Antifoulings used to be a copper base type coating. Copper is actually a poison that leeches in small doses. That is one reason that you never see a lot of build up of organisms in copper pipes, as anything that attaches to it dies. This was how AF worked. If a barnacle or anything else attached to it - dead. Apparently, some regulation was passed and prohibited paint manufacturers from using copper in the AF coating. Now most of them modify epoxy based or vinyl based coatings to create surface conditions that are too "slick" for marine growth to get a good grip on, so that it is hard for them to attach to the hull. The other thing they do is make "ablative" coatings. These coatings are designed to wear off the hull. Some of these also have a small amount of microbiocide to help keep growth off the hull. What is supposed to happen is that as the vessel passes through the water that whatever is growing on the hull creates resistance and is pulled of with a very thin layer of the ablative coating. In my opinion, they work at best fair, and only if your service speed is over 10 knots.

 

[berkshire]

mshimko (Materials)
This is not a subject I want to dwell on; I start getting upset every time I have to deal with it.
The boats we are discussing here are sailing yachts belonging to a sailing club in San Diego Ca.
I and the boatyard are getting hit from both sides; the solvent based antifouling paints are being phased out because of VOC regulations. The ablative paints we used to use have too many Voc's for the yard to use. Although if I want to put a boat on a trailer and take it to my house and paint it that’s legal, go figure.
The other thing is the water quality management district, which is saying that due to poor flushing in some of the marina basins, the level of dissolved copper is too high and even as we speak is trying to enact legislation to ban copper biocide. On a voluntary basis we are trying to reduce the amount of copper in the basin.
I have one boat painted as an experiment with Ceramcoat 54
It is now 3 years into its life and is holding up well, however it does have to be cleaned every two weeks instead of once a month, so it does not save me any money.
Anyway thanks for letting me vent.
B.E.

 

[kenvlach]

berkshire,
FYI, I believe your Feb. 4 diagnosis of 'electrolytic corrosion' is basically correct.
However, it's not from stray electrical currents but due to galvanic corrosion between the iron matrix (anodic) and graphite (cathodic). Known as graphitization or graphite corrosion:
“In this case, the iron matrix corrodes, leaving behind a porous graphite mass that can be carved with a pocket knife.” Significant weakening can occur w/o significant dimensional change.

It only occurs in low corrosion situations, e.g., under deteriorated paint. Flowing seawater exposure causes more uniform attack of both iron and graphite (& sometimes pitting).

An eddy-current technique can be used to detect and measure graphitization.

Your paint pretreatment is reasonable given the conditions. If permissible, the best pretreatment would be abrasive blasting plus chromic acid treatment.* Given the porosity, it's probably best that you avoided other acids or hydrowashing. Pretty hard to rinse and dry. Platers generally alternate hot & cold DI water to cleanse the porosity, and powder coaters bake dry before painting.

For future reference (new keels): Graphitic corrosion only occurs in gray cast iron with its graphite flake network. Nodular (ductile) and malleable cast irons don't have the problem. For maximum corrosion resistance in saltwater, consider an alloyed C.I. In 15-year testing in Panama, an 18% Ni C.I. had a weight loss only 1/12 that of gray C.I. at mean tide level and ¼ in continuous immersion (Pacific Ocean). The lowest corrosion rate I found listed for continuous seawater immersion was 0.0008 inch/year for a cast iron of composition 3%C-1.2Mn-1.7Si-6.5Cu-15.0Ni-2.0Cr (wt%) tested 6 years at Kure Beach, North Carolina.
This composition corresponds to ASTM A436 Austenitic Gray Iron Castings, Type 1 and ASTM A439 Austenitic Ductile Iron Castings, Type D-2. Interestingly, some of the test bars in these specifications are called 'keel blocks.'

Iron Castings Handbook, pp. 317-321 (1971) and pp. 492, 497-502 (1981).
“Development of a Cast Iron Graphitization Measurement Device,” NYGAS Technol. Briefs, Issue 99-690-1 (Jan. 1999). [the eddy-current technique]
ASM Handbook, vol. 13 Corrosion p. 566-572 (1987); updated vol. 13B pp. 43-50 (2005).

*How about shipyards in Tijuana and if not, Ensenada?

Funny(?) that the water suppliers can still use copper sulfate to kill algae in reservoirs.

 

[berkshire]

kenvlach (Materials)
A big thank you for your response. You have described a form of corrosion I was unaware of. Your description is perfect."Iron matrix corrodes, leaving behind a porous graphite mass that can be carved with a pocket knife.”
You are suggesting a chromic acid rinse to produce FeCr2O4), rather than Phosphoric acid producing iron phosphate?
As an experement I had a keel unbolted then sandblasted and shipped to a powder coater who put on a heavy coat of epoxy. This was done a year ago, so it has not had enough time in service to see if this is the way to go.
There will be no new keels. These boats are thirty years old, the patterns are long gone. Unless I make one.I don't think so, the last pattern work I did was in 1962 and I do not have an expansion rule anymore.
Regarding your comment about yards in Ensenada this is happening more and more here. The two major yards in Ensenada are getting far more work from the US than they used to because of the environmental restrictions.
B.E.

 

[mshimko]

Thanks for the info, both of you (berkshire & ssreese, and I need to pay attention to whom other posts are aimed at

The San Diego area is a pretty "extreme" area regarding both air and water emissions. Thus far, AF paints are in a specialty category in the NESHAP regulations; but there was a reporting requirement a few years ago (and I assume it still exists) that could indeed make it very difficult for a marina to apply a perfectly legal AF paint!

I finds it interesting that you (berkshire) are using Ceramkote 54. I looked into this coating (or may a previous version of it) about 10 years ago when preparing a feasibility study on AF paints for the Navy. What you describe is pretty much what I recall back then - cleanings would be required every couple of weeks, whereas the AF paints the Navy uses now need cleaning only every 3-4 years.

Regarding "easy-release coatings, other biocide coatings, etc - we've yet to find any that even comes close to the current generation of copper ablative coatings.