Ship Hull Preservation

Pardon my ignorance but I was under the impression that cathodic protection systems only dealt with galvanic and stray current corrosion. Simple corrosion such as rust is an oxidation process. How does cathodic protection stop oxidation?

In order to halt rusting of iron based steel plate you need a barrier coating to prevent contact of oxygen molecules with the iron molecules.

Coatings are a good choice, but sacrificial anodes (often magnesium) electrically connected to a buried steel tank, say, will erode away leaving the steel intact. That kind of corrosion is usually due to the action of acidic soil components. In alkaline soils, this isn't so necessary because that kind of soil doesn't usually mobilize metals.

The original poster said "The hulls are made of carbon steel. The hulls have been in the salt water for 35+ years and corrosion has made them thin".

Nowher did it say anything about being buried in acidic soil. Hence my question regarding how cathodic protection stops simple iron oxidation. During 10 years of marine electrical work which includes a fair amount of stray current corrosion detection and prevention, not to mention galvanic corrosion protection I have never heard of any claims that cathodic systems prevents simple rust from forming. Hence my question of how cathodic protection will prevent a steel hull immersed in salt water from rusting.

Sacrificial zincs are used in the vicinity of propellers and rudders not to mention some other dissimilar metal fittings to stop galvanic corrosion. High water velocity areas around the prop must be protected because galvanic corrosion activity increases with water velocity. Prop shafts and propellers are dissimilar metal from hull plates.

Docked ships are not subject to such activity. They do rust from simple oxidation of the steel plates.

Coatings are a good choice, but sacrificial anodes (often magnesium)....

For fresh water magnesium is used, for salt or brackish water zinc is used.

Travis

Elnav,

You are right that "Simple corrosion such as rust is an oxidation process." It involves transfer of electrons as the iron is oxidized and some other material is reduced.

Therefore, think of the ship's hull as one plate in a very large-scale electrolytic cell, with the ocean water being the electrolyte and the other "plate" being either O₂ dissolved in the water or the earth. As the iron gives off two or more electrons to become the ions Fe⁺⁺, or Fe⁺⁺⁺ etc. in the various compositions of iron oxide (rust), there is a voltage produced between the ship's hull and the other plate of this "electrolytic cell". In the absence of anything to force the reaction backwards, the more electrochemically stable form is the oxide. As in any other electrolytic cell, if you apply to the plate/terminal a voltage higher than its own output voltage this will drive the reaction the opposite way, or at least stop the reaction if it is not physically/chemically reversible. In carbon/zinc cells, the nature of the oxidation reaction products does not permit it to be reversed; hence they cannot be recharged. I suspect the same is generally true of the ship's hull.

Therefore, applying a DC potential sufficiently larger than the ~0.7 volts that comes from the oxidation of the iron has the theoretical ability to stop the oxidation. As in so many other areas, the devil is in the details: how much current, how much over-voltage, what type of plate to use for the other side of the "cell", safety to people, safety to aquatic life, etc. Apply too much voltage and you generate H₂ gas.... I personally don't know if this has been done because of these and many more "details", but the theory is valid.

Virtually every element has a certain oxidation/reduction potential for it to gain or lose electrons. The electrolytes certainly vary, as do the potentials involved. Hence, a lithium ion battery has a greater voltage per cell than a lead/acid battery..... All we are dealing with here is a cell in which Fe (metallic) is one of the two electrodes.

--John M.

think of the ship's hull as one plate in a very large-scale electrolytic cell, with the ocean water being the electrolyte and the other "plate" being either O2 dissolved in the water or the earth. As the iron gives off two or more electrons to become the ions Fe++, or Fe+++ etc. in the various compositions of iron oxide (rust), there is a voltage produced between the ship's hull and the other plate of this "electrolytic cell. REPLY Good point! In the cathodic protection systems I have installed or repaired, th eprotection goal is to shift the potential of the metal to be protected by -200 mV closer towards the zinc value of 1200 millivolts. I have never seen a steel hull which did not contain other metal fittings. Therfore I cannot draw on my own experience for corroboration of your hypothesis. But if I understand your argument correctly, a plate of mild steel should not oxidize even when immersed in salt water provided the galvanic potential is shifted by that 200 millivolts. The practical problem of monitoring this potential lies in how to reference the O2 molecules without introducing a third metal in the form of a cathode.

I agree with what Jim says. The cathodic protection is probably your best defense against corrosion. Ensure the zinc's are still in place and have not completely dissolved away and that you have current flowing to these. Additionally make sure the vessel is properly grounded so that current flows to all points in the hull.

For city / industrial water pipes a "sleeve" was developed that could be inserted into the pipe to stop leaks and extend the life of the pipe. For boats parked outdoors they now have plastic shrink-wrap coverings, the space shuttle has tiles on it's surface, ...

Because these ships are "parked" what alternatives might that bring to mind, that would not normally be used, much like a steel building in salt water.

"If I had eight hours to chop down a tree, I'd spend six sharpening my axe." -- Abraham Lincoln

The WWII battleship USS Alabama had similar issues. What they wound up doing was cofferdamming around the ship and pumping the water out. This gave them access to the hull for patching and painting. From what I know, it's worked out very well for them.

On the zinc plates, you might check into the active anti corrosion systems that use an electric current to reverse the problem. They would know what these systems can and can't do. You may even get them to use some used or re manufactured systems for the bragging right that they have their systems on your ships.

I know government and cost savings don't mix

Just an Idea

Brad

Interesting comments,

I was on the USS Alabama about March, 2008 and the reason they built the cofferdam around the ship was because of a hurricane, the ship had been sitting in the mud (or coffee grounds as any sailor knows), after the hurricane the ship was listing, so they built the coffer dam, flooded to float the ship and reset it level as I recall. Thus concrete only around the hull might help to stabilize, although concrete floats too if the waves get high enough. There is a US submarine up on dry land at the USS Alabama museum beside US Hwy 10 and a German WWII sub in Chicago at the Museum of Science and Industry.

In the hydropower industry, the non-rotating parts of the turbines and the inside of the water penstocks are usually painted with coal-tar epoxy after the metal has been sand-blasted to a white surface.

Yesterday I sent an email to Corrpro, who builds and installs corrosion protection equipment, Corrpro bought Good-All Electric in the late 1990's, I had sold Good-All Electric battery chargers to electric utilities at one time, the Good-All Electric founder was among the first to build corrosion protection equipment.

It seems there should be a passive way, that doesn't require electricity ?

Site dependent maybe parking the ships on dry land in a concrete slip is a least expensive option. The mooring fee's at a pier may also be significant.

Passive anti-corrosion is usually in the form of a coating. For recycling credits, check this out:

< www.faqs.org/patents/app/20080196619 >

I believe there are tar epoxies which can be applied to a wet surface, and some sewage treatment plants have used these for on the fly repairs to coatings.

Contact the makers of tar epoxies and they can probably advise you.

Having spent some time in the Navy (20+ years), I can tell you that there is cathodic protection, mainly zincs installed that get changed on a regular basis. Hull inspections are done by divers on a regular basis and they are changed as needed. Ships also go into drydock on a regular basis of every several years where the hulls are sandblasted clean and repainted. Most of the ships built are designed for a 30 year +/- lifespan.

From the sounds of it most respondent have spent time around ships. We have all seen various examples of the different protection schemes. However unless you actually work in that specialty the subtle distinctions may escape you. The original question was about protective barrier coatings to protect the carbon steel shell plating. Cathodic protection is done to protect dissimilar metals in close proximity. Zincs have an effective radius of maybe 20 - 30 feet and that is a stretch. For longer distance you need an active impressed current system. If you paid attention when you ship was in dry-dock you will recall the majority of zincs were placed on the rudder blade, on the hull close to the prop and possibly around prop and rudder shafts as a collar. These are the areas of fastest water velocity and thus need the most protection. Some fittings like the older sound domes and sonar emitters may have been bronze and would also have zincs. A few more were placed at 50 foot intervals along the hull These were never intended as whole hull protection but to counter the effects of any dis-similar metals used in thru hull fittings. Zincs are use in areas of greater water velocity. The galvanic table is referenced to a specific water temp and velocity. My buddy and mentor worked as the corrosion specialist in San Francisco Bay area and did not only the aluminum hulled Fast Ferries but also all the old wooden ships in the maritime museum. He was a dealer for CAPAC who makes active impressed current systems and he also did a lot of passive zinc protection work. Cathodic protection is not effective for hull shell protection as a whole. It is intended for specific metal areas of limited dimensions. Barrier coatings must still be used to protect hull plating. Present technology for hull plate protection in general is to use an epoxy based coating (paint) that is applied dry. This is both a corosion protection and anti fouling coat. These days the paint coatings are expected to give five year protection. As you mentioned sand blasting is a required prep step since nothing else will remove all oxides and moisture traces. In the small craft world of yachts and pleasure boats we do on occasion use high tech stuff that does work underwater ands will adhere to wet non prepared surfaces. This is strictly for leak stoppages and would be prohibitively expensive for a complete hull coating. In-situ applied coatings are not going to be effective since they would end up being applied over any fouling and barnacle growth. As someone else noted, the enzymes found in barnacle glue can also attack the steel plating. And exclusion of oxygen on stainless steel will cause crevice corrosion.

Try this company. http://www.jotun.com/us I assume you have already checked the more common and better known companies like Petits, International Paints and Interlux. Jotun is more of a commercial ship paint provider and not so much for smaller quantity as is used for small craft like yachts under 100 feet. Jotun delivers paint in 55 gallon drums.