Corrosion: Immersed Steel Plate

[sloop2]

I thought I would ask the professionals about steel corrosion at it relates to the effects of seawater on hull plating. I ran across the attached information in the ABS technical papers (see link; item #19, page 207) that gave me pause in considering the longevity of steel hulls. If I am reading the study correctly, it would suggest that non-linear wastage of steel plates accounts for roughly 2.5mm at 12 years and nearly 4.0mm at 24 years. Does this study ring true for real-world conditions? Are steel hulled boats re-plating every 12-24 years just to keep up with the effects of corrosion and wastage?

http://www.eagle.org/news/TECH/TechPapers05.pdf

[timgoz]

Mild steel plate, properly blasted & coated, and with the coating system properly maintained, will last indefinetly.

Diligent, but not nessasarily overwhelming, maintainance is a definent requirement for steel boat ownership.

A well designed, built, and maintained steel boat should have a 50+ year life.

Take care.

TGoz

[mastcolin]

These figures sound about right to me. People like NACE have details on average loss for every environment eg freshwater-immersed, industrial non-immersed, urban non-immersed etc.

The point you missed, as answered, is that this is unpainted. You'd be daft not to not paint steel that was immersed or not.

As answered, properly painted steel will last almost indefinitely. If there are only minute levels of moisture and oxgen permeating the coating, the corrosion rate is very, very slow. SS Great Britain is 150 year old or something. The Queen Mary is 50 years old. The US Navy still has WW2 battleships mothballed. Even the Titanic is 100 years old and it's permanently immersed (I don't recommend this experiment on your boat), old fashioned paints on low quality steel with limited surface prep I'd imagine

[mallo]

Hi Sloop
Having family owned and sailed a steel sailing boat built in 1939 (68 Years) since 1973 there is very little wastage providing a good coating is applied (she is looked after) there is an infinite life for a steel hull, we had a major refit two years ago, (long story won’t go there in this post) we had thickness testing through the whole hull prior to some steel repairs and the main hull was still the original thickness. The only areas that were thinned were low down in the bilge where water stood or could stand the wastage was only form the inside. Providing zinc anodes are fitted on the outside even where the protection is damaged the anodes protect the steel and wastage doesn’t occur.
The wastage in the bilge hasn’t happened recently, I think probably happened in the early years, partly due to the war(I think the boat was layed up during this time and was probably forgotten) and paints weren’t what they are now…..
One thing to note is there wasn’t any wastage under the engine bilge (a certain amount of engine oil in the water??????)
Providing the hull is properly sandblasted and coated up it lasts for many years with just the occasional touch up on any paint chips…..
A lot of people think steel as a material for boat building a lot of work to keep the dreaded rust at bay, but looked after and properly cleaned and painted they have an infinite life, the surveyor at the end of this last project said the boat is in better condition now that before, a lot of this is down to the modern coatings.

[riggertroy]

Currently working on a McDermott Built AHTS - she's just been through her 4th Special survey, plating in way of the keel coolers and chain lockers is the only plating that has needed renewing, being worked hard most of her life and the underwater plating getting cleaned and painted every five years shows that the steel will last if protected, the plating inway of keel coolers wasted due to the heat and difficulty in maintenance, the chain lockers from the inside out due to lack of paint, new steel has now been coated with an epoxy paint, will see how it copes with the chain bashing it, would rather ave wood clad the lockers but I didn't have a say.

[mallo]

Hi riggertroy
I agree with what you say re needing protection, that’s the main cause of rusting, lack of cleaning and painting. Chain lockers have been re-plated (65 years of rusting and cleaning and re-painting…..) sand blasted inside, epoxy coated and then glassed over with epoxy resin based fibre glass to try and “protect” epoxy paint from the chain chips…… will see how we get on in time….

[PI Design]

The UK MoD has a DefStan which specifies the corrosion margain that the designer must allow for. I'm not sure if the values themselves are classified or not, but the annual corrision is quite a low percentage of the steel thickness - similar to the ABS figures you quote. Typically the ships are designed for a 30 year life, and even with the very thin steel used on warships the majority of the hull does not corrode to anywhere near the minimum allowed. Problems mostly arise in bilges where the water is contaminated, and in other areas where the steel is not permanently submerged (and can therefore rust if not coated well). The biggest single corrisive is sulphate reducing bacteria (SRB). This occurs in contaminated fuel supplies and I think it is their excrement (if bacteria do that?!), which is acidic and eats the metal. If this occurs corrosion can be quite rapid and problematic (5mm in just a few years).

[PI Design]

We use a value similar to the ABS figures you quote. Typically the ships are designed for a 30 year life, and even with the very thin steel used on warships the majority of the hull does not corrode to anywhere near the minimum allowed. Problems mostly arise in bilges where the water is contaminated, and in other areas where the steel is not permanently submerged (and can therefore rust if not coated well). The biggest single corrisive is sulphate reducing bacteria (SRB). This occurs in contaminated fuel supplies and I think it is their excrement (if bacteria do that?!), which is acidic and eats the metal. If this occurs corrosion can be quite rapid and problematic (5mm in just a few years).