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The following articles on Corrosion in Salt Water Marine Engines
have been submitted by Mark Stetch from Australia (thank you Mark...).
He has some preliminary comments as follows:

Corrosion in Salt Water Marine Engines - Part 1

by Mark Stretch © copyright retained

Part One – What is corrosion?

The material contained within these articles has been gleaned from a number of sources. It should be noted that I am not a chemist and as such some of the chemical formulae may be “lacking”. I have not tried all of these processes in fact only the mechanical, molasses and electrolytic treatments.

Some of the processes for corrosion removal that are discussed may use high temperatures, electrical current or dangerous chemicals. You should not undertake any of these unless you are aware of the risks and dangers. Notwithstanding the information is provided “as is with no regard to the completeness, accuracy or results”. If you choose to use any of this information then you assume all risks and responsibilities for this.

I have spoken to several people about some of the chemicals discussed. They are mentioned on the basis that they are more likely to be commonly available, more easily handled, less dangerous and can be properly disposed of. There are similar chemicals available which may be much more effective but have significant drawbacks. If you are a chemist then you would know this, if you are not then I would not experiment.

Seawater is pretty much a chemical soup. As such it is very difficult to accurately predict all of the possible chemical reactions and other processes that occur during the corrosion of cast iron engine parts exposed to seawater. The following is a description of the gross phenomena of corrosion of cast iron in seawater.

It is mostly concerned with the corrosion of internal water passages as this is where most of the problems occur.

The presence of moisture and oxygen lead to the corrosion of iron. The salts within sea water, notably sodium chloride, mean that the type of corrosion is different and more damaging than the type of iron rust that is commonly seen on land based or freshwater iron engines.

Seawater is a very harsh environment for iron objects. They will corrode 5 times faster in the sea than in freshwater and some 10 times faster than air of normal humidity.

The corrosion and resultant by products is more damaging in enclosed areas such as water jackets for the reasons discussed below.

The most common type of corrosion is called electrochemical. Basically any piece of iron is effectively an electrochemical cell much like a battery. It has anodes and cathodes.

The anodes and cathodes are caused by dissimilar metals,impure iron(most metals are not pure they are alloys of some type), stress points and the minute amounts of dissolved metals present in all seawater. The differences in oxygen levels, temperature and the pH at the surface of the iron means that electrochemical cells will form even on completely pure iron

Electrons flow from the anodic area to the cathode causing metal to corrode by way of the formation of positive ions at the anode. It should be noted that these cells are at a microscopic level. Typically there may be many millions of these cells within a water jacket. They only stop forming and causing corrosion when equilibrium is reached or the metal is completely oxidised.

The following illustrates a typical reaction

At cathode the more noble

2H2O + 2e >> H2 + 2(OH)

These hydroxides then combine with the sodium ion in the seawater to produce sodium hydroxide

Na+ + OH >> NaOH

At the anode is the production of ferrous ions

Fe – 2e >> F+2

These combine with chloride in the seawater to form ferrous chloride

Fe+2 + 2Cl >> FeCl2

Ferrous chloride is a corrosive mineral acid. When exposed to oxygen it oxidises to ferric chloride and ferric oxide

Ferric chloride is commonly used to etch copper electronic circuit boards. Ferric oxide (Fe2O3) is another oxide of iron. FeO is the red rust we commonly see.

When combined with the cathodic product sodium hydroxide they may form ferrous hydroxide.

Often salt-water engines may sit for long periods of time with salt water in the jackets. After a while the above process(and maybe the anaerobic process described later) has used all of the available oxygen in the seawater.

What then happens is that corrosion continues up until ferrous chloride stage. As well there are localised changes in the pH of the water which results in the precipitation (coming out of solution) of calcium carbonate and magnesium hydroxide. These intermix along with the ferrous chlorides, sand, debris, rust organic material, etc to form hard deposits around the metal.

Cast iron also undergoes a process called graphitisation. Within the structure of the iron are flakes of pearlite and graphite. The pearlite is an anode and the graphite a cathode. Eventually the pearlite corrodes leaving a porous structure that is filled with the corrosion products discussed above. This leaves a weak structure that will deform easily.

Once the salt water is removed from the water jacket and air is allowed to enter the corrosion process will continue or even intensify unless precautions are taken. Remember within both the hard deposits and crystalline structures of the corroded iron are chlorides. These chlorides will start to oxidise on exposure to air. This can occur at quite a rapid rate.

This oxidisation causes them to expand and occupy a greater volume. As well the residual chlorides which are held in solution will crystalise and expand as they dry out. This is very similar to spalling you sometimes see on brick walls.

It explains why a cylinder, which looks perfectly ok, may crack and/or deform soon after it is removed from salt water and allowed to dry.

There is another type of corrosion called anaerobic corrosion. I don’t think that this is the major type of corrosion that we see however it can play a part, especially in engines that have sat in salt water for a long time without use.

The hard corrosion deposits around the metal can to a degree slow down or protect the underlying metal from further corrosion. However within the deposits can live sulfate reducing bacteria. These bacteria are commonly found in salt water.

They require an oxygen poor or anaerobic environment (decaying organic material, seaweed and the chemical reactions discussed above can provide this environment within a water jacket) as well as sulfates. Sea water contains sulfates.

The bacteria produce hydrogen sulfide as a by product. This also corrodes the iron. I’m also told that the hydrogen use by the bacteria has an adverse (worsens) effect on the electrochemical corrosion process.

So there you have it as well as corrosion we also have bacteria eating our engines!

Any successful strategy of restoring an engine affected by seawater corrosion must address the following:

·         Proper storage and handling of the cast parts prior to any work on the restoration.

·         The removal of any deposits.

·         Either render the residual chlorides inactive or remove them as much as possible.

·         The repair of cracks and holes (if there are any).

·         Treatments to protect the restored areas from future corrosion.

·         If the engine is to be used in a boat then cooling methods other than salt water i.e. freshwater, proprietary cooling solutions etc.

Storage and Handling

Ideally iron parts, in an unrestored state, should be stored in an inhibitive solution. An inhibitive solution solution prevents the further corrosion of metal as well as excluding oxygen.

It would seem that there are a number of strategies:

·         Leave the engine where it is (if it is in a boat) until you are ready to restore it.

·         Store completely submerged in fresh water.

·         Store in oil, kerosene, anti-freeze or similar.

·         Utilise a true inhibiting solution.

Whilst all approaches have some merits and all are better than letting an engine sit in a corner and dry out it would seem that a true inhibiting solution is the best option.

There has been mention of using anti-freeze as the storage solution. At this stage I am unable to determine whether typical anti-freeze is an inhibiting solution. I do not think it is, as the high pH levels required of an inhibiting solution would remove paint if spilled.

Research indicates best inhibiting solutions have a high pH. Marine archeologists typically use a 5 percent sodium carbonate solution (soda ash). This has a high pH and will passivate (stop further corrosion) the iron. However the engine should be disassembled prior to immersing in the solution with only iron parts put in the bath.

It has the approach of using easily available materials(soda ash and fresh or distilled water).

There are some precautions using soda ash. It will corrode aluminium. Parts should be free of oil. It will remove most types of paint. Eye protection and gloves should be worn as it is alkaline.

If using this approach the pH will need to be monitored and adjusted. If the pH is allowed to drop too far further corrosion will result. The pH should be kept above above 10 but below 13(below pH 10 and above pH 13 further ACCELERATED corrosion will result). This is most easily achieved by changing the solution if the pH falls outside the chosen range.

Parts should be completely immersed ie do not let any part of the casting protrude above the solution. If you do rapid corrosion will most likely take place at the air-solution interface.

Storage in sodium carbonate solution should be considered a temporary solution. Objects should not be stored in the solution for long periods of time(ie many months to years).  It is meant protect the cast iron parts prior to the restoration process.

Procedure for short to medium term storage of cast iron engine parts:

1)       Clean the parts thoroughly. Remove as much flaking and encrustation material as possible by brushing, scraping, chipping etc under fresh running water.

2)       Degrease the object. I have found commercial degreasers work Ok. Rinse the parts in mild soap solution followed with fresh running water.

3)       Prepare the solution. It is 5% which is by weight.eg 950grams of water to 50grams of soda ash. Will make just under a litre of the solution. OR

For non-metric water weighs 8.33 pounds per gallon so for 10 gallons of solution the total weight is 83 pounds the weight of sodium carbonate is 83 /19 = 4.4 pounds.

4)       Place the parts in the bath.

5)       Monitor the pH and keep in the range 10-13. Note the mixture will initially be pH @11.5.

6)       Change solution weekly

7)       After two to three months the solution changes can be extended to monthly.

Please dispose of the used solution with regard to the environment. You can use a mild acid(swimming pool shop) to reduce the pH to 9 or below and pour down the drain.

I was given a box of pH strips. They appear to be readily available form chemical supply houses at very modest cost (they are considered a consumable item in the chemical business).

I am told that there is an added benefit to this process in that it will also soak out some of the residual water soluble chlorides. It should be noted that some of the chlorides are not water soluble.

The next article will discuss various treatments for deposits and the residual chlorides including:

·         Mechanical removal.

·         Electrolytic reduction.

·         Galvanic removal.

·         Molasses Bath.

·         Alkaline Sulfite.

Setups, procedures and materials will be discussed for each method.

Part 1 | Part 2 | Part 3 not available yet