Ship board Corrosion

spongebob
22nd October 2009, 08:40
Ship board Corrosion

We are all familiar with the need for cathodic protection on ship’s hulls and although I have almost forgotten all about it I do recall corrosion being beyond the norm on one ship, the little banana boat “Navua”
She was only a couple of years old when an attempt correct the loading list by flooding a deep tank had no effect and it was discovered that the diaphragm plate dividing the port and starboard tanks had gained a large hole in the form of a lacy lot of corrosion to allow both tanks to equalize.
This happened before I joined the ship and by the time I did the Company had had the Auckland Department of Scientific Research involved and after plate repairs each tank had a long tear shaped anode suspended from the tank top to counteract the problem.
We had to periodically remove the tank access doors to check the protection and I recall the anodes being replaced a couple of times.
Other problems raised their head such as the time we were pumping up the after peak tank to correct the loading trim and a kind waterside worker came on board to tell us that water was pouring out of a hole in the hull a few feet above the water line.
Same problem, an isolated patch of filigree corrosion which was repaired and similarly protected but we were beginning to wonder if there were any other areas under threat. The ship had a going over in dry dock at Port Chalmers and all seemed well but then yet another bogey reared its head in the form of cylinder liners eroding/corroding on the water side to damage the sealing ‘O’ ring landings and allow water to leak into the cylinders.
A pulled liner looked like it had been attacked by toredo worm on the water side and initially the powers that be, the Marine Supers, elected to fill the thousands of shallow cavities with a metallic gunk but after one trip this was failing and the problem remained.
I was only the third engineer doing the donkey work but I did read the manual issued by Sulzer Switzerland to cover an engine built under license on the Clyde and it stated that the liners were “Fescalised” on the water side surfaces. To this day I remember that word but not the true meaning and a look on the net now suggests that it might mean ‘machined’ or ‘plated’
I left the ship soon after and a year later found myself on the Tarawera with an identical design engine but built by Sulzer Switzerland and the spare liners had a dull plating not unlike cadmium to look at so I assumed the Clyde licensee had overlooked that requirement.
I never did hear of the final outcome but the way the corrosion damage was occurring I suspect that Navua had to have new liners well before reasons of wear.
Perhaps a long winded techo-post but someone from the engine room dept might have had a similar experience.

Bob

Andy Lavies
22nd October 2009, 10:31
Hi, Bob,
Cathodic protection is used to counteract corrosion caused by dissimilar metals in an electrolyte (seawater) usually steel hull and bronze propellor. I wonder what other metals there were in the deeptank to set off the rot. May just have been poor quality steel or poor initial coatings.
Andy

eriskay
22nd October 2009, 11:29
As Andy Lavies says, cathodic protection, either by impressed current or sacrificial anodes, is the normal method employed to deal with the problems associated with dissimilar metals of differing electrical potential in a commom liquid - seawater in this case - becoming the electrolyte, and leading to galvanic corrosion if not corrected.

Whereas in the hull/propeller situation an accelerated rate of corrosion attack would take place if not dealt with, in the deep tank scenario, where it is unlikely that different materials are employed in the fabrication, a reduced rate of corrosion would nevertheless take place on normal ferrous surfaces, unless protected by coatings/linings. These do not totally indemnify against corrosion however and require regular inspection and maintenance to maintain their protective characteristic and offset natural wear and tear.

Protective coatings are fine and well under highly controlled application conditions, where cleanliness, surface preparation and ambient conditions can be maintained to the specified conditions. The fabrication and construction profile of ship construction is a particularly difficult application to achieve this. Internal and external corners, shape steel stringers, gussets, mouse-holes, branches, nozzles, etc, are notoriously difficult to blast and prime/paint to the required standards and invariably it happens that small areas break down in service. When that happens, an accelerated and concentrated attack can be anticipated to follow in these areas.

Delamination between the substrate and protective layers is another problem area that will lead to the corrosion pattern described by Bob Jenkins as 'lacy/filigree'. Crevice corrosion is another risk area, and in some carbon steels (as used at one time by the Japanese shipyards) the Cu content is higher than normal and any release by dissolution can lead to a phenomenon known as 'copper cell mechanism' corrosion attack, but this is more associated with applications where heat is involved and the released copper cell gets trapped between the substrate and, say, some attaching scale. When that happens it can penetrate thick carbon steel plates very rapidly until it exhausts itself. A rare situation but can happen.

The cylinder liner application involved heat and salts therefore any surfaces in contact, especially under shutdown/stagnant conditons, should be in a material that is highly-resistant to these arduous conditions. There are many forms this can take, some of which are better than others, but for the avoidance of problems the more noble the sprayed-on metalling protection system the better as it may have to deal with temperature, sulphites, chlorides and other nasties.

Satanic Mechanic
22nd October 2009, 12:05
Just as a side note there Eriskay - modern building practice has all corners either radius-ed or heavily chamfered allow proper coating. Coated ballast tanks still have zinc anodes fitted aluminium being preferred for the external hull.

eriskay
22nd October 2009, 13:30
SM - thanks for that.

As an aside, was installing some Desalination Plants in Middle East (1970s) that included a system of impressed current cathodic protection in the watertube boxes. Heard the Contractor referring to the little Armenian guy who was setting up the cathodic protection system circuits as 'The Archbishop'. Intrigued I queried what was the reason and was told it was because his limited English referred to the system as 'Catholic Protection' and no one could convince him the correct word was 'Cathodic'. So, they started referring to him as 'The Archbishop' !

Billieboy
22nd October 2009, 16:27
I've run into several unusual corrosion problems, a few high temperature corrosion of stainless steel valve spindles in way of gland packing area, these took the form of grain boundary corrosion due to graphite in the packing materials, when looked at closely, it gave the impression of, "worm-holes", in the stainless steel. The corrosion usually took 15-20 years before the spindle was reduced enough in strength to renew it.

Another, most unusual, corrosion case was still water ionic corrosion in stainless steel 316 water valves in a distillation plant. The valve in question was a drain valve where the body and lid were of stainless steel 316, the water was distilled water without any impurities and in that position, non-moving. It seems that an ion movement started up and within two years a hole was corroded in the valve box. Careful examination and study, solved the problem by changing the valve spec to stainless steel 316L. The result was an extremely happy plant Chief Engineer.

The best of all unusual corrosion stories was the zinc-oxide type tape used to attach production numbers onto pipe used for nuclear power generation steam plant, where this particular bit of steel was irradiated, causing the residual, "glue", to change it's molecular form and attack the steel of the pipe. This one didn't happen on my watch, I just read about it.

chadburn
22nd October 2009, 17:10
This is where the old saying comes to bear when it comes to painting/coatings whether it is a ship or the front door "Failure to prepare is to prepare for failure". When the RN laid their steam jobs up they cleaned the boilers out renewed the zinc plates and filled the boilers with distilled water

cubpilot
22nd October 2009, 18:58
regarding the cylinder liners, any glavanising treatment of the coolant system presuposes that the the coolant will not have any chemical water treatment. since most shipowners go for treated water then corrosion would not be expected. should a coolant system have galvanised pipes etc and someone then decides to add treatments no end of trouble occurs as the system sludges up from the chemicals/zinc reaction.

spongebob
22nd October 2009, 23:47
Thanks for all your comments,we all understand the cause and effects of electrolytic action and resulting corrosion but in the case of the ship I highlighted it had a sister ship "Karamu" and two near relatives "Kaimai" and "Konini"
All were built by Henry Robb, all had the same Sulzer trunk piston engine built under license in Scotland but to the best of my recall only "Navua" suffered this continual problem with both the hull and the engine.
I can tell you we got sick and tired of pulling liners every time we got home but other than that she was a great little ship.

I am reminded of the time at school when in the Electricity and Magnetism class we had a lesson on wet cell batteries, anodes, cathodes , electroplating etc.

I went home into Dad's workshop, filled Mum's earthenware bread making bowl with a solution of dissolved nickel sulphate crystals then using an EPNS spoon as an anode and my older sister's large gold plated but dulled bangle as a cathode I attempted to give it a silver sheen.

Connected across a 6 volt battery the bubbles soon started and the bangle turned black.
I got both physical and audible corrosion in the ear hole from my mother and sister but I though I caught a gleam in my father's eye as he thought "That's my boy"

Bob

Philthechill
23rd October 2009, 11:26
I've run into several unusual corrosion problems, a few high temperature corrosion of stainless steel valve spindles in way of gland packing area, these took the form of grain boundary corrosion due to graphite in the packing materials, when looked at closely, it gave the impression of, "worm-holes", in the stainless steel. The corrosion usually took 15-20 years before the spindle was reduced enough in strength to renew it.

Another, most unusual, corrosion case was still water ionic corrosion in stainless steel 316 water valves in a distillation plant. The valve in question was a drain valve where the body and lid were of stainless steel 316, the water was distilled water without any impurities and in that position, non-moving. It seems that an ion movement started up and within two years a hole was corroded in the valve box. Careful examination and study, solved the problem by changing the valve spec to stainless steel 316L. The result was an extremely happy plant Chief Engineer.

The best of all unusual corrosion stories was the zinc-oxide type tape used to attach production numbers onto pipe used for nuclear power generation steam plant, where this particular bit of steel was irradiated, causing the residual, "glue", to change it's molecular form and attack the steel of the pipe. This one didn't happen on my watch, I just read about it.Billieboy, g'day! Strangely enough I was waiting 'til I'd read through this "Thread" before adding my five cents worth about "strange" ways stainless-steel can corrode.

I was going to relate the story I heard when I was working at Sellafield on a big water-chilling system the firm I worked for (UDEC Ltd.) had installed.

At the time I found the yarn hard to believe but seeing this addition of yours concerning how the "glue" attaching zinc-oxide ID tape, to s/s steel pipework, can have its molecular structure altered, by being irradiated, it's more than likely the story I was told was true after all.

There was a huge hold-up at one time, (not exactly a rare occurence at Sellafield!!) when it was discovered that a particular type of felt-tipped pen, being used by some of the welders to put their ID on welds, was on the "banned list" of substances that, on no account, were to be used on s/s pipework which would be exposed to radiation, as it would promote corrosion. Nobody knew, with 100% certainty, which welds had the banned felt-pen ID and which didn't so, to be on the safe side, EVERY weld done by that particular contractor had to have the felt-pen ID cleaned-off and the site of the ID polished to be certain there was no felt-pen residue left.

I was promised a look-see at the "banned list of substances" by a BNFL friend but it never happened.

Incredible though how innocuous things, used in everyday life, which cause no problems normally can alter after being bombarded with neutrons and take on a completely different persona. Cheers for that one Billieboy! Salaams, Phil(Hippy)

greektoon
23rd October 2009, 12:04
One place where you have to be wary of electrolytic corrosion is on accommodation ladders at junctions of aluminium alloy and steel components. I have found joints / bolts not properly insulated and the resultant wastage can and does cause failure of the structure. Very dangerous.

Billieboy
23rd October 2009, 18:58
One place where you have to be wary of electrolytic corrosion is on accommodation ladders at junctions of aluminium alloy and steel components. I have found joints / bolts not properly insulated and the resultant wastage can and does cause failure of the structure. Very dangerous.

Probably lack of metallurgical education in the local drawing office, used to get the same problems with aluminium accommodation ladders and the lifting and fixing attachments. All that's needed is either a copper earthing wire or bakelite/plastic washers and sleeves. I've found that it's no good talking to ship's staff, one has to beat it into the heads of the owners and the builders.

cubpilot
23rd October 2009, 21:02
mention of stainless steel pipes reminds me of the building of a 'government ship' i was involved in. all the seawater pipes for the sanitary system were in stainless and had to have a special weld method. cost the shipyard in time and money. in service all was fine for a while but after a year the welds started to drip then give way. cost the yard even more. over specified and could have been done just as well in glavanised and at commercial rates. been talking today with a man involved in the building of part of these new aircraft carriers. again over specified on standard systems and will no doubt have similar teething problems and so costs will soar even more.

hamishb
24th October 2009, 01:02
On the British Might in early 1961 we had a strong and persistant smell of gas on the deck.
The chief decided that the whesso valves must be leaking so we spent many days dismantling ,cleaning and grinding in every one of the valves.
The smell was pretty strong while we were working at the valves but the chief was convinced that once we were finished all would be OK.
After completion of the job and the next day the smell was as bad as ever.
The chief went on the hunt again and eventually discovered the cause of the niff.
The spare propellor wad fitted on the foredeck on a pedastal and underneath the prop the deck had vanished.
A cement box was fitted around the prop boss and that kept the emell at bay.
No worries as we went to the breakers after discharging the cargo
A real old workhouse of a ship but happy with it.

eriskay
24th October 2009, 02:08
mention of stainless steel pipes reminds me of the building of a 'government ship' i was involved in. all the seawater pipes for the sanitary system were in stainless and had to have a special weld method. cost the shipyard in time and money. in service all was fine for a while but after a year the welds started to drip then give way. cost the yard even more. over specified and could have been done just as well in glavanised and at commercial rates. been talking today with a man involved in the building of part of these new aircraft carriers. again over specified on standard systems and will no doubt have similar teething problems and so costs will soar even more.

Normal quality stainless steel material in seawater applications is not suitable and will fail in due course, and particularly fast in the event of stagnant flow conditions (pinhole corrosion attack). Galvanised steel would be even worse, I'm afraid. For seawater or brine applications a super-dulplex stainless steel alloy is the solution, albeit an expensive one, or, if the pressure is low enough, one of the plastics or FRP/GRP would be less costly by far. The problems don't end with the use of super-duplex St St, however, as this is notoriously fickle in terms of welding, where strict control of the weld procedures is required, and careful pickling and passivation of the heat affected zones.

The Gloom and Doom Merchant ! (Cloud)

johnb42
24th October 2009, 02:41
We ran across a problem with cathodic protection in the DB's in one of the CP Ships Nelson/Calgary class. The ship was on a regular trade carrying prefabricated houses from Montreal to North Africa. Since the ship never reached her marks with this cargo, the DBs were constantly left full. The cathodic protection system had been designed on a loaded voyage/ballast voyage basis, with the DBs empty for half the time. With the tanks left permanently full, the anodes lasted only half the time they were meant to and the steel work of the floors and the shell began to corrode.
In a Company down by the head with Naval Archs no one picked this up until advanced corrosion was noted.

Satanic Mechanic
24th October 2009, 23:15
One place where you have to be wary of electrolytic corrosion is on accommodation ladders at junctions of aluminium alloy and steel components. I have found joints / bolts not properly insulated and the resultant wastage can and does cause failure of the structure. Very dangerous.

Aluminium on deck equipment - I spend a large part of my life talking slowly saying the words "I don't care if it is good in sea water its still attached to a large lump of steel"(Cloud)

Another one from left field is the increasing use of titanium plate coolers - they can have the damned annoying habit of turning the rest of the system into a very large sacrificial anode.

funnily enough i have just came off a job with pitting in the cargo tanks - 6000 repairs, blasting and coating - just shy of a 7 figure repair bill - ouch.

Billieboy
26th October 2009, 11:56
Standard problem with coated tanks, saw that starting up in late '80s, dink the coating with anything and three weeks later there's a fine pit, getting bigger and deeper every day! drilling holes in ships bottoms without even trying. Gets me thinking if the cork and fishing line trick, for when arivet used to fall out! Trouble is, the 5/8" and 3/4", bolts don't come already drilled!