Keel Corrosion / Rust

[Harry Brown]

So, just hauled the boat out today and found some spots of surface corrosion on the Keel.
I would like to remedy this correctly before she is relaunched.
Now, I only have 6 days to do this. is this feasible within this time frame??
If so, would you you guys be so kind as to tell me what materials I would need and the procedure used to do this?
If this is not possible in the time scale, would sanding it back, priming and antifouling and leaving until next year, be acceptable?.
I would really like to sort properly though.
Thanks for all help in advance.

[dawntreader]

Whilst waiting for a full answer you may want to look at this post: http://www.bavariayacht.info/forum/index.php/topic,1765.msg10645.html#new

[MarkTheBike]

Harry, if it's any help - I had a much worse case when I hauled out after purchase 3 yrs ago. Not knowing much about how to fix (and not having a lot of time), I cleaned the several areas THOROUGHLY with a drill and wire-brush cups and discs, filled with 2-part epoxy, cleaned the bare (but good) metal surrounding epoxy again and carefully applied 2 coats of primer (Primocon, from memory but open to correction). Prime it carefully and inspect/fix any pinholes in both coats. That means inspecting each patch closely! Then 2 coats of antifoul and not had any leaks, weeps or stains since. It was done as a stop-gap originally but has survived very well. It should do you until you have a longer haul-out. Good luck.

[Kibo]

I had similar corrosion to Mark on my keel after just 2 years.

Thoroughly sandblasted, applied primer then 4 coats of epoxy with fairing followed by antifouling. I was also advised it might be caused by galvanic corrosion through current leakage in the marina so as a belt and braces approach I had two small anodes fitted to the keel one either side by welding on threaded studs. Nearly one year down the line I can see no repeat of rust when snorkeling under the boat. I am about to haul out next week for the season so will be able to inspect antifoul more closely.

[MarkTheBike]

Yes, I would have sandblasted too but I didn't have enough time to organise it, and the drill/wirebrushes did a decent enough job (and kept the rust at bay while the epoxy set). I should add that the 2-part epoxy was a marine grade filler rather than a coating. I wondered about galvanic corrosion at the time but discovered that keels don't actually suffer from it, it's just good old-fashioned rust where the paint integrity has failed.

[Ricd]

Harry

The few rust spots look pretty normal and not at all serious for an iron keel. In short term I would rub off the rust, wet the keel and apply a rust convert or such as Fertan, leave 24 hrs, brush off black dust that results, prime the spots with Primocon and re antifoul. The keel is not going to rust away.

[Kibo]

WRT possible galvanic action I found this reference in a similar thread from the US. With cited sources.

 I will inspect my keel anodes next week and report back on any deterioration too.....

This following copy and paste is from the thread:

http://www.eng-tips.com/viewthread.cfm?qid=146280


"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. 13Bpp. 43-50 (2005)."



Ian
 

[Harry Brown]

Thank you for all of the replies.
In the process of spot treating, priming, then antifoul.
Was a bit worried when I first saw it! Thanks for putting my mind at ease!

[Impavidus]

Harry.
Hi. You will need to have the keel blasted back to bare metal. grinder, flappy wheels and sanding discs just will not open the pores in the casting.
Cast Iron is porous to a certain extent so salt water will be below the surface, which is why you need to have it sand or soda blasted.
The next issue with cast iron is once basted it will flash corrode on the surface in a matter of hours (rust) after blasting.
Therefore it is important to coat or protect as soon after blasting as possible.

So the best action is to scrap back and remove as much fouling or paint as possible a couple of days before your blasting takes place. This will give the iron time to breath out any salt water. Then pressure wash at a really high pressure to remove any salts. then a couple of days later blast with sand or soda back to the metal.
When the blaster has done, get him blast the keel all over with his high pressure air line to take off any dust or residue. You may find that your casting has hollows or ripples in it after blasting.

This is not unusual. You will need to fill and fare these out using an epoxy based marine grade filler. (DO NOT USE CAR BODY FILLER) Preferably a non solvent based one as the solvent can take days to leach out after curing. (that is why epoxy smells for days after its hard).

You now have a couple of choices. The first is to prime with an epoxy primer like Seajet 117 Epoxy Primer. Then 4 coats of epoxy paint, something like Gel-shield and finally your anti foul.

Or I have used a Polysulphide primer and 4 layers of top coat on a couple of boats with fantastic results. No rusting years later. I get it from a place in Southampton. I can not remember their name but they are down on the waterfront near the stadium. You then apply the antifoul.

Of course if your blasting the keel and spending a couple of hundred pounds doing a proper job you may want to think about copper coat all over. It pays for its self in about 3 years and if you do the work yourself the cost of a 36 footer is less than £100.00 for the materials..........

Hope this helps.

Ant.       

[Salty]

............ Prime it carefully and inspect/fix any pinholes in both coats. That means inspecting each patch closely!

Checking for pinholes in the paint coatings at shipyards in Japan during the late 60s and early 70s, we used a paint brush to which an electric cable had been connected and which was also connected to a low voltage buzzer such as a battery operated  doorbell. The buzzer/doorbell was in turn connected to a nearby and as yet unpainted part of the ship's hull. In use the paint brush was dipped into some fresh water before brushing over the area being checked. Any pinholes would allow water through to the steel, and allow the circuit to be completed, causing the buzzer to sound. It was a very effective tool, and more reliable than the human eye !!

[Symphony]

Harry.
Hi. You will need to have the keel blasted back to bare metal. grinder, flappy wheels and sanding discs just will not open the pores in the casting.
Cast Iron is porous to a certain extent so salt water will be below the surface, which is why you need to have it sand or soda blasted.
The next issue with cast iron is once basted it will flash corrode on the surface in a matter of hours (rust) after blasting.
Therefore it is important to coat or protect as soon after blasting as possible.

So the best action is to scrap back and remove as much fouling or paint as possible a couple of days before your blasting takes place. This will give the iron time to breath out any salt water. Then pressure wash at a really high pressure to remove any salts. then a couple of days later blast with sand or soda back to the metal.
When the blaster has done, get him blast the keel all over with his high pressure air line to take off any dust or residue. You may find that your casting has hollows or ripples in it after blasting.

This is not unusual. You will need to fill and fare these out using an epoxy based marine grade filler. (DO NOT USE CAR BODY FILLER) Preferably a non solvent based one as the solvent can take days to leach out after curing. (that is why epoxy smells for days after its hard).

You now have a couple of choices. The first is to prime with an epoxy primer like Seajet 117 Epoxy Primer. Then 4 coats of epoxy paint, something like Gel-shield and finally your anti foul.

Or I have used a Polysulphide primer and 4 layers of top coat on a couple of boats with fantastic results. No rusting years later. I get it from a place in Southampton. I can not remember their name but they are down on the waterfront near the stadium. You then apply the antifoul.

Of course if your blasting the keel and spending a couple of hundred pounds doing a proper job you may want to think about copper coat all over. It pays for its self in about 3 years and if you do the work yourself the cost of a 36 footer is less than £100.00 for the materials..........

Hope this helps.

Ant.     

What you describe is a good process. BUT it is an alternative to regular patching which is what most people do. What you describe is very labour intensive and simply not economic if you pay someone to do it, neither is it 100% effective. It would make sense if you are Coppercoating the whole boat, but if you are antifouling annually then makes much more sense to do local patching every year. Almost zero cost and not a big chore if you keep on top of it.

[Impavidus]

Symphony, I agree "you pays your money" as they say . If your happy to keep patching its not an issue.
On our last Bavaria a 2003/32 we started getting rust patches when she was about 4 years old. I had Symblast blast the keel and we used the Polysulphide paint with good results for the remaining years we had her. It was about £150.00 for the blasting and £100 for the paints. I think we used to pay about £375 for a lift and block off to anti foul and change the anodes at our marina.
Once we had done the keel we were able to use the sea-lift or dry out on piles at Fareham to do the same tasks in a hours. Both were a much cheaper option for us at £200.00 and £50.00 respectively.

Best regards.

Ant

[Impavidus]

I just found these..........

Ant.

[Kibo]

As promised here are photos after haul out. Anodes show some slight wear so maybe there is something in the galvanic theory ? No rust on the keel so it shows it is worth doing a proper job of sandblasting priming and base coat prep work as discussed above

[Symphony]

Quite common to get a small amount of galvanic action on a cast iron keel because the iron will not be pure, but will have impurities in it. However their proportion compared with the bulk of iron is very small so you rarely see any evidence of it. It has no real impact of rust as that is the result of water and iron. So if your keel is well protected from water you won't get rust. The challenge is achieving that level of protection.

[Kibo]

Thanks Symphony

Seems the yard that re coated my keel did a very good job. I'm very happy to see no rust at all after a full season in the water since it looked pretty bad this time last year