Hull corrosion

[Midnight]

Here's a puzzle I helped a friend two-pack his hull sides and base-plate last year. Today he dry-docked to see how it was doing and was quite shocked to see the hull sides covered in silvery flecks but nothing at all on the base-plate. Both were painted just after the sand blasting and loose sand removed. He has an isolation transformer installed. 

 

 

 

 

Edited September 12, 2020 by Midnight

[jonesthenuke]

We inspected the hull of our boat last year after doing 2 pack three years before (we wanted to dry dock to do other painting, add some anodes and do some repairs) There was some loss of coating around the anodes (expected) but none elsewhere except for an area where we had obviously scraped the side on something. 

 

If the coating problems shown above are widespread I suggest discussing with whoever did the job. How good was the original preparation? 

 

Out of interest, what coating material was used?

[Alan de Enfield]

22 minutes ago, Midnight said:

Here's a puzzle I helped a friend two-pack his hull sides and base-plate last year. Today he dry-docked to see how it was doing and was quite shocked to see the hull sides covered in silvery flecks but nothing at all on the base-plate. Both were painted just after the sand blasting and loose sand removed. He has an isolation transformer installed. 

 

 

 

 

 

 

 

That look as if it could be Type 2 Microbial Corrosion (not 'normal' corrosion due to electrical 'leakage')

 

'Wider pictures' show similar random silver 'pits' in the blacking.

 

 

Boat owners and yards know all about rust. There is endless literature on electro-chemical and galvanic corrosion – all under the general heading of ‘rust’. But there are other types of corrosion which closely resemble (but are not) rust in the conventional sense about which little is known by boat owners and by many yards. This is a corrosion caused by microbiological action which is can occur on boat hulls, particularly those lying in canals or rivers containing high levels of chemicals or decaying vegetable matter. Microbially Induced Corrosion (MIC) is a highly unpredictable process but under the influence of micro-organisms, corrosion processes can be rapid, happening in a matter of months compared to the years it would take for ordinary abiotic corrosion to reach serious proportions. This phenomenon is well known in the oil, gas, water and mining industries but is little understood in the steel boating world. MIC frequently occurs in areas with high nitrate content in the water – this particularly pertains to arable regions of the canal network and particularly to canals and rivers on the east side of the UK and where there is intensive crop farming using non organic chemical fertilizers with consequential phosphate, sulphate and nitrate run-off into the watercourses. Marinas fed by rivers are another risk area and, in salt water environments, it is well known that harbour muds are highly contaminated by sulphides produced by these creatures. Sulphide films are, by their very nature, highly corrosive and the identification of such very obvious. It is usually found under muddy and slimy surfaces, sometimes even behind paint coatings and a very careful visual inspection is necessary to locate it. It is not discoverable by non-destructive testing such as ultrasonic thickness measurement, eddy current testing or the magnetic method familiar to most marine surveyors. The bacteria are often found inside oxidised welds or in areas which contain physical defects such as porosity, overlap or lack of penetration. The microbes leading to this condition can both cause corrosion from beneath existing coatings or seek out pinpricks in the steel coating and cause the reaction to occur from the outside. MIC bacteria can be present under previous blackings and is not eradicated by simple pressure washing. Unless correctly treated, MIC can continue to thrive beneath the coating, emerging as major pitting.

 

 

 

 

 

[Tracy D'arth]

Hulls shedding coatings around anodes is a known phenomenon, maybe because the steel sheds mill scale. Another reason for grit blasting.

[mark99]

Disbonding near sacrificial anodes is due to..... racks brain........can't remember the name. Hydrogen bubbles are formed on parent metal skin if excessive galvanic current present which forces coating off. I think it's called overpolarisation or something. That's why on impressed CP current systems more protective current is not a silver bullet. The steel coating disbonds by H2 forcing it off. Ironically if the coating was 99% and no anodes the coating would not disbond due to H2 bubbles as H2 bubbles would not form.

 

This could be a case of what I keep banging on about...... great coating but pinholed concentrates the galvanic loss.  Did you say baseplate was coated too?

Edited September 12, 2020 by mark99

[Captain Pegg]

1 hour ago, Midnight said:

Here's a puzzle I helped a friend two-pack his hull sides and base-plate last year. Today he dry-docked to see how it was doing and was quite shocked to see the hull sides covered in silvery flecks but nothing at all on the base-plate. Both were painted just after the sand blasting and loose sand removed. He has an isolation transformer installed. 

 

 

 

 

 

I don’t see any significant active corrosion in this photograph.

 

There is clearly pitting in the top left but it appears to be covered with the hull coating suggesting it is historic. There is a small amount of surface discolouration to centre and lower left but it’s a red/brown colour and we know the base material beneath the failed coating is steel. No mystery there.
 

What is evident is a silvery surface material where the coating has failed but I can’t see any evidence it has eaten into the parent steel unless the owner has reported otherwise. I don’t think it’s simply the base steel because it is unlikely to remain in its raw state and in any case the colour appears too silver rather than grey.

 

Therefore we need to think about what happens when the coating fails in proximity to the anode which two members who between them have relevant experience of both boats and the corrosion of metals in wet environments say is a known phenomenon.
 

If you have an anode then by definition who must also have a cathode (your anode is just a lump of metal otherwise). The cathode is the exposed steel hull around the anode. Electrolysis is a process whereby chemical reactions occur at the electrodes. We can see the evidence of this in the anode by its depletion. But where does the depleted material go?

 

Are then the silvery flecks the chemical product of electrolysis? I don’t claim to actually know but my guess would be that they are and that they are some form of magnesium (hydr)oxide.
 

I’d be willing to bet a vastly larger sum of money on that than it being microbial corrosion, because I literally can’t see it.
 

JP

Edited September 12, 2020 by Captain Pegg

[dmr]

I believe this sort of thing is pretty normal near to the anodes, we have it. I don't understand it though, if the epoxy is 100% waterproof I don't see why or how the steel takes part in any reactions.   It looks like you have something similar a bit further away from an anode which is odd, but it ain't that far away. If you use the boat you will get lots more scrapes in the epoxy that will need periodic repair, don't worry about it.

 

.................Dave

[Captain Pegg]

8 minutes ago, dmr said:

I believe this sort of thing is pretty normal near to the anodes, we have it. I don't understand it though, if the epoxy is 100% waterproof I don't see why or how the steel takes part in any reactions.   It looks like you have something similar a bit further away from an anode which is odd, but it ain't that far away. If you use the boat you will get lots more scrapes in the epoxy that will need periodic repair, don't worry about it.

 

.................Dave

It clearly can’t be if we are observing a reaction on the surface of the steel.

[Keeping Up]

29 minutes ago, Captain Pegg said:

If you have an anode then by definition who must also have a cathode (your anode is just a lump of metal otherwise). The cathode is the exposed steel hull around the anode. Electrolysis is a process whereby chemical reactions occur at the electrodes. We can see the evidence of this in the anode by its depletion. But where does the depleted material go?

Indeed. In normal electrolytic activity the material from the anode is transferred to the cathode; hence are we here just seeing pieces of silver anode material on the surface of the hull?

[mark99]

18 minutes ago, dmr said:

I believe this sort of thing is pretty normal near to the anodes, we have it. I don't understand it though, if the epoxy is 100% waterproof I don't see why or how the steel takes part in any reactions.   It looks like you have something similar a bit further away from an anode which is odd, but it ain't that far away. If you use the boat you will get lots more scrapes in the epoxy that will need periodic repair, don't worry about it.

 

.................Dave

No coating is 100%. Only coating salesman think - assert that! 

 

Edited September 12, 2020 by mark99

[Midnight]

1 hour ago, mark99 said:

Disbonding near sacrificial anodes is due to..... racks brain........can't remember the name. Hydrogen bubbles are formed on parent metal skin if excessive galvanic current present which forces coating off. I think it's called overpolarisation or something. That's why on impressed CP current systems more protective current is not a silver bullet. The steel coating disbonds by H2 forcing it off. Ironically if the coating was 99% and no anodes the coating would not disbond due to H2 bubbles as H2 bubbles would not form.

 

This could be a case of what I keep banging on about...... great coating but pinholed concentrates the galvanic loss.  Did you say baseplate was coated too?

No that's what's puzzling the base-plate is untouched

[Captain Pegg]

7 minutes ago, Midnight said:

No that's what's puzzling the base-plate is untouched

Not really since it will be passivated by years of slow oxidisation and likely a build up of freshwater mussels. It won’t be that reactive compared to the freshly exposed areas where the coating has failed. Those areas are also nearer to the anode so the electrolytic cell formed will be stronger.

[Midnight]

34 minutes ago, Captain Pegg said:

It clearly can’t be if we are observing a reaction on the surface of the steel.

Again this is a puzzle. Although about 30 years old and not in tip-top condition the hull and base-plate were sand-blasted prior to the two-pack coating being applied just one year ago. It's exactly the same system I've used on Midnight which despite being 16 years old now has no such issues. Although the base-plate was perhaps given an additional coat the hull sides had two coats applied. I think the  Type 2 Microbial Corrosion mentioned by Alan de Enfield and Captain Pegg is the answer just very surprised it happened in such a short time span and now need to find out what the correct treatment is. In the past I remember someone washing the hull sides with Domestos.

Edited September 12, 2020 by Midnight

[Captain Pegg]

13 minutes ago, Midnight said:

Again this is a puzzle. Although about 30 years old and not in tip-top condition the hull was sand-blasted prior to the two-pack coating being applied just one year ago. It's exactly the same system I've used on Midnight which despite being 16 years old now has no such issues. Although the base-plate was perhaps given an additional coat the hull sides had two coats applied. I think the  Type 2 Microbial Corrosion mentioned by Alan de Enfield and Captain Pegg is the answer just very surprised it happened in such a short time span and now need to find out what the correct treatment is. In the past I remember someone washing the hull sides with Domestos.

I did not suggest it is microbial corrosion, quite the opposite. Please go back and read my post again and if it’s not clear I’ll amend it.

It’s only a puzzle if you ignore the evidence.

 

The system you used is almost certainly fit for purpose since it’s presumably formulated for the purpose but the application is key. Take note of what @mark99 said and also accept the application of the coatings will have been variable in the environment and perhaps the mixing of the 2-pack. On the latter occasion for whatever reason it appears it has been sub-standard.
 

 

Edited September 12, 2020 by Captain Pegg

[Midnight]

1 hour ago, Captain Pegg said:

I’d be willing to bet a vastly larger sum of money on that than it being microbial corrosion, because I literally can’t see it.
 

JP

Apologies I misread your last sentence

[Captain Pegg]

4 minutes ago, Midnight said:

Apologies I misread your last sentence

No worries, my posts are rarely brief but to summarise:

 

- the coating has failed locally in the vicinity of the sacrificial anode

- what you observe is not corrosion of the hull but the resulting product of the sacrificial anode doing its job

Edited September 12, 2020 by Captain Pegg

[Midnight]

For clarity the silvery spots are all down the hull sides not just on the vicinity of the anodes. My initial thoughts were galvanic corrosion but as I said the boat has an isolation transformer fitted.

[Captain Pegg]

2 minutes ago, Midnight said:

For clarity the silvery spots are all down the hull sides not just on the vicinity of the anodes. My initial thoughts were galvanic corrosion but as I said the boat has an isolation transformer fitted.

What is happening beneath them?

[Alan de Enfield]

1 hour ago, Midnight said:

Again this is a puzzle. Although about 30 years old and not in tip-top condition the hull and base-plate were sand-blasted prior to the two-pack coating being applied just one year ago. It's exactly the same system I've used on Midnight which despite being 16 years old now has no such issues. Although the base-plate was perhaps given an additional coat the hull sides had two coats applied. I think the  Type 2 Microbial Corrosion mentioned by Alan de Enfield and Captain Pegg is the answer just very surprised it happened in such a short time span and now need to find out what the correct treatment is. In the past I remember someone washing the hull sides with Domestos.

I have a number of documents that tell you why it appears under coatings and, how to get rid of it but I cannot post them here as Pdf's are not allowed, happy to email them to you if you send me a PM with you address,

 

Basically :

 

If a hull is found with evidence of microbial attack, it is necessary to deal with it to try to prevent it recurring.

A simple solution is for the whole area to be washed with copious amounts of high pressure fresh water. When dry the area affected should be coated with a strong bleaching agent (sodium hypochlorite) diluted 1:4 with water and left for twenty four hours.

 

Afterwards a second high pressure fresh water wash is necessary followed by recoating. This will probably remove around 90% of the microbes but the only real solution is to blast back to bare steel and to treat any inaccessible areas such as tack-welded rubbing strakes as best one can with the bleach solution before applying the next stage of the coating process.

The main problem is that the microbes can continue to live beneath the existing paint coatings and once sealed in with a fresh blacking, the lack of oxygen and light is the perfect environment for them to thrive leading to a risk of corrosion from the inside out.

 

No coatings are entirely proof against a microbial attack from the exterior. Minute pinpricks, mechanical damage below the waterline are all opportunities for the microbes to penetrate the steel and commence the process from the outside in..

 

WARNING SODIUM HYPERCHLORITE IS HIGHLY CAUSTIC AND TOXIC. IT MUST BE TREATED WITH GREAT CARE AND RUBBER GLOVES, WELLINGTON BOOTS AND EYE SHIELDS ARE ESSENTIAL.

 

FINAL FINISHING

If the pressure washing has exposed areas of bare steel, it is recommended that a zinc phosphate rust prevention system such as Fertan be applied. This should be allowed to work over a 24 hour period and MUST be thoroughly washed off with water and a brush to ensure that only the bare steel retains the Fertan before a top coat of Keelblack is applied.

This is essential to ensure that any subsequent coating is properly attached to the hull.

 

Nevertheless, the microbes can still live underneath adjacent prior paint coatings so the only certain way to remove the risk of future attacks is by blasting back to bare steel – an expense many owners may not wish to contemplate.

 

 

 

[dmr]

32 minutes ago, Midnight said:

For clarity the silvery spots are all down the hull sides not just on the vicinity of the anodes. My initial thoughts were galvanic corrosion but as I said the boat has an isolation transformer fitted.

Was the hull well cleaned after shotblasting, or could some residue have remained and mixed in with the epoxy?

 

I assume the epoxy has failed and those spots are on the steel rather than on top of the epoxy????

 

How extensive is the problem?, what %age of hull is showing this problem? and is the distribution random or a clue?

 

............Dave

[Midnight]

51 minutes ago, dmr said:

Was the hull well cleaned after shotblasting, or could some residue have remained and mixed in with the epoxy?

The residue sand was brushed off before coating. I didn't bother with that when I did Midnight just mixed it in and it's been okay.

51 minutes ago, dmr said:

I assume the epoxy has failed and those spots are on the steel rather than on top of the epoxy????

I've only seen the photos but asked the question and was told it's on the steel

51 minutes ago, dmr said:

How extensive is the problem?, what %age of hull is showing this problem? and is the distribution random or a clue?

 

............Dave

It's all over except for the base-plate and seems to be random

[dmr]

I dunno, was the photo taken right after pressure washing? The silver could be shiny steel that has not yet rusted as the epoxy was taken off by the pressure washer (again possibly not unusual but not as bad as this). Was the epoxy put on with a roller and not driven right into the pits in some places? or just not fully mixed?

 

................Dave

[Richard10002]

17 hours ago, jonesthenuke said:

If the coating problems shown above are widespread I suggest discussing with whoever did the job. How good was the original preparation? 

 

As far as I can tell, it was a DIY job by the OP and the owner.

[jenevers]

On 12/09/2020 at 21:41, Captain Pegg said:

Not really since it will be passivated by years of slow oxidisation

That's something I've not heard of. Can you elaborate please?

[Captain Pegg]

7 minutes ago, jenevers said:

That's something I've not heard of. Can you elaborate please?

Steel rusts because it’s surface in its natural state is chemically unstable. Once it has formed an oxidised layer on the surface it is less reactive and further rusting occurs at a slow rate. Rust is a protective layer in itself albeit not a fully effective one. Hence the effects observed on this boat appear to be more pronounced on the raw steel exposed by the localised failure of the coating on the hull compared to the effects on the baseplate despite the fact the baseplate has not been deliberately protected.

Edited September 13, 2020 by Captain Pegg