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Corrosion in Salt Water Marine Engines - Part 2
by Mark Stretch © copyright retained
Part Two - Corrosion Treatment
To recap on the previous section the process of corrosion in salt-water marine engines is quite complex. Salt water affected cast iron must be properly handled and stored otherwise further accelerated corrosion is inevitable.
From all of the available evidence:
1) DO NOT let the part dry out.
2) EXCLUDE oxygen from the part OR store in an inhibiting solution that passivates the iron.
Whilst each part will be different in the approach taken a proper plan should be made. If you are using one of the chemical based methods discussed it may be helpful to make detailed notes for future reference.
There are five possible or used methods of treatment
* Mechanical removal.
* Electrolytic reduction.
* Galvanic removal.
* Molasses Bath.
* Alkaline Sulfite.
I'm sure there are others.
We know from part one of these articles that the corrosion process results in two main types of products being the corrosion we see (the oxides) and the metal salts (chlorides) we may see as part of the hard deposits and which may be held within the corroded areas and the top layers of partially corroded metal.
We are all familiar with the process of oxidisation i.e. red rust. Oxidisation is simply the formation of a new compound by the addition of oxygen. There is also the opposite of oxidisation called reduction. Oxidisation cannot happen without reduction occurring somewhere else. Several of the types of treatment use reduction where the casting is the cathode and a sacrificial anode is used.
Reduction has the advantage of reversing the corrosion process back to the original metal. Please note that it is only some of the metal not all.
However any successful restoration must also address the issue of removal of chlorides as well. In fact as we saw earlier the chlorides allow the corrosion process to continue unabated. The oxides are the end result of the corrosion process.
Only a few treatment methods address the chloride issue. All of them excepting the mechanical cleaning method still leave the problem of the hard deposits. If the casting has been allowed to dry out deposits may become softer after either the electrolytic cleaning or alkaline sulfite treatment.
If after treatment we have reduced or eliminated the chloride levels and stabilised the corrosion then we may have the opportunity to use further chemical treatments on the hard deposits.
These treatments may include hydrochloric acid, glycolic acid or one of the proprietary formulations for cooling system hard water or furnace treatment. Once the deposits have been removed it will likely be necessary to reprocess the item to deal with the chlorides which have been “locked up” under the hard deposits.
Otherwise mechanical means and the cutting of access ports may be used. If this route is taken then it may be better to do the mechanical treatment before using one of the chemical processes to treat the chloride problem.
This method appears to be the most widespread of the corrosion treatment processes used by marine engine collectors.
Various tools such as scrapers, picks, chisels, steel cables in drills etc are used to remove the hard deposits. Holes are often cut into the water jacket to reach hard to access areas. The holes are then sealed by welding, brazing, epoxy etc
This is the method I have used up until now. It does clean the casting up and seems to be quite successful. It does not really address the chloride issue, however various solutions of anti freeze, oil, proprietary solutions are kept within the water passages after restoration to limit corrosion.
It would seem that even though corrosion may often continue the removal of the deposits allows the new corrosion to grow into the now free space without leading to problems such as cracks and distortion.
However without further treatment for the chlorides the corrosion will continue and at some point in the future the casting will likely suffer cracking, deformation or holes.
Galvanic cleaning has been discussed as a possible method of removing corrosion deposits.
It involves placing the casting in a bath and surrounding it with an anodic metal such as aluminum foil. The bath is then filled with a 20% solution of soda ash.
The casting is then left in the bath until the aluminum has dissolved.
This process is not recommended as base metal is always removed from the casting, it is not a reduction process, and it is very difficult to check the progress as the casting is surrounded by the aluminum foil.
The same result could probably be achieved by use of acid. The acid depending on the type would also make some inroads into the hard calcium deposits. It too would have the result of removing base metal, which is something to avoid as we are trying to conserve as much metal as possible in the casting.
Electrolytic Reduction Cleaning
This method is the one of two of the preferred methods used by marine archeologists.
It involves setting up an electrolytic cell with the casting as the cathode. The cathode and a suitable anode are placed into a bath with an electrolyte. Current from a DC power supply (+ to anode and - to the casting) is applied.
What happens is that oxidisation occurs at the anode. Positively charged metallic ions flow travel to the casting. At the casting reduction takes place where hydrogen is released and some of the positively charged ions on the casting are reduced to metal.
Depending on the current strength the hydrogen will mechanically clean the casting and remove any completely oxidised deposits.
As well chlorides are drawn out of the casting and travel to the anode.
You will need some equipment and materials for this method. They are:
1) Power supply. A modified battery charger can be used but for best results you will need a variable power supply of from 6-24v
2) Anode Material. Mild Steel rods, mesh or plates.
3) Clips and wire. Steel alligator clips. If they are plated then the plating should be removed by soaking in weak hydrochloric acid. Wire should be multi strand two ply as used for home appliances.
4) 5% solution sodium carbonate.
5) A method of determining chloride levels.
There are some disadvantages to this method in relation to marine engines. Firstly as chloride is removed from the casting the levels within the solution rise. This can cause new areas of corrosion. Secondly carbonates can precipitate out of the solution and form as microscopic deposits on the metal effectively stopping the chloride removal process.
However the major problem with this method is that for optimum results it requires a form fitting anode i.e. the anode must be able to “see” the corrosion. In the case of water passages this would mean that the anode would have to be within the water jacket for best results.
The method does have some merit for solid objects. I recently used this method on a some small solid parts with good results. However in hindsight it would have been simpler, although slower, using the molasses method.
There may be some application where, say a cylinder is completely blocked if quite a few holes were cut and the anode placed where the corrosion can be “seen”. Another option could be several small anodes, wired together, used in water jacket passages.
If you wish to experiment with this method then I would strongly urge you to forgo the battery charger route and buy or build a proper power supply. For consistent results you will also need some method of measuring the chloride levels with the solution (part three of this article).
Parts should be degreased, with paint and any loose deposits removed. Later on in the process the hydrogen bubbles will remove paint and loose scale but for the initial processing we want the maximum rate of chloride reduction. Loose deposits or paint will slow down the chloride removal
The process should start off at a low current. For best results it would seem the power should be adjusted until hydrogen bubbles appear on the surface of the casting and then backed off until the bubbles disappear. I am lead to believe that this will cause the maximum removal rate of chlorides.
Once the chloride levels within the solution have risen and then leveled off a new batch of solution should be used. Initially tap water can be used but when the chloride levels within the bath drop to the levels of chloride within tap water deionised water should be used for the next batch of solution.
I am told that we should be aiming for a chloride level <50ppm. At this point the current can be increased till hydrogen bubbles appear. These bubbles will mechanically clean the surface.
When the process is complete the casting should be rinsed several times in distilled water and a protective coating applied immediately.
1) do not let the anode and cathode touch.
2) do not reverse the polarity. If you do the casting will start to corrode, rapidly. Experiment on some expendable items first.
3) if you increase the power too much the iron will undergo a process called hydrogen embritlement.
4) if the wires become hot they are too small. Use a larger gauge of wire.
5) You cannot treat more than one item at a time.
This is also a reduction method. I am unsure as to how it works. However it would appear to have very little effect on chloride. It may be that over a long period of time (many months to years) it may reduce the levels of water-soluble chlorides.
The method is very simple. Add the casting to a mixture of water (80% - 90%) by volume and molasses (the remainder 10%-20%). In fact the ratio is not critical I’ve heard of people getting good results from 2 to 1 mixes all the way up to quite weak mixtures.
The part must be free of oil (the process will not work on oil coated areas), again a commercial degreaser works well. It is best to have a lid as various insects and other animals will be attracted. You will know when the process has started (a couple of days or so) as a foamy surface will start to develop. Depending on the extent of the corrosion the complete process will take from a few days up to several months.
In the winter I use a small submersible aquarium heater to speed the process up. Unfortunately the hard deposits seem largely unaffected by the molasses bath, they still need to be manually removed.
One you are happy with the results the casting should be washed in water and a protective coating applied.
Molasses can be easily obtained from your local farm or livestock supply company. The mixture can be reused, although I would be mindful of chloride levels in mixtures that have been used many times.
Alkaline Sulfite Treatment
This method is the other of the two preferred methods used by marine archeologists. I have not tried this process nor do I know anybody whom has.
It does appear to meet the requirements of both oxide treatment and also chloride removal.
I have not specifically given the recipe for the solution as the chemist I speak to considers the one I have as dangerous to use in a non-laboratory situation.
He is working on an alternative solution which is reasonably safe to use in a non-laboratory setting. This will be posted when finalised.
Basically the object should be cleaned and degreased. It is then immersed in the solution in a sealed container and heated to 60 degrees Celsius.
The casting passes through several baths of the solution. The chlorides are monitored and the process is continued until they have been removed.
Solutions should be made up, monitored and changed at the same frequency as the electrolytic reduction cleaning. It would seem that this method is a lot quicker than the other chemical methods discussed.
This process converts the iron corrosion to magnetite and resolves the chloride problem. Magnetite offers some protection against further corrosion.
Again it requires chloride level monitoring. It seems to require a miniumum of equipment, an air tight container and some method of keeping the solution heated(restaurant and catering equipment auctions?).
Although I have not used this process I think that it would work well on water jackets as it does not rely on corrosion being “seen”.
There are some other treatments which fall under the heading of annealing. They involve heating the casting to high temperatures in a reducing(pure hydrogen or mixed hydrogen helium) environment in a furnace at very specific temperatures and for very specific times.
This reduces some of the corrosion back to base metal and also melts the chloride salts out of the casting.
They are outside the scope of any home or even industrial workshop I have ever seen. Obviously heating hydrogen to high temperatures (remember the Hindenburg) is fraught with danger. As well the by-products such as hydrochloric acid which end up on both the casting furnace are problematic in themselves.
If you have something that is very rare or valuable it may be worthwhile enquiring with a large marine artifact conservation museum. They may have facilities for this process or can point you to someone who has. The process takes several days so it will likely be expensive.
There is another method called “direct flame” I have very little information on this but I will post when I have all of the details to hand. It appears to be similar to the various annealing processes but is done with a special type of flame and possibly a shielding gas in open air.
Irrespective of which method is used the chloride levels must be monitored and reduced, so the next article covers chloride level measuring techniques, alkaline sulfite recipe, direct flame details and anything else of interest.
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