Workboat

Maintenance

How To Control Corrosion

In Your Boats

Specially written for PIM by LEN SPINDLER, technical director of Sample Marine Products Pty. Ltd., Sydney.

I can’t help being amazed at the apparent mystery sur rounding the word electrolysis in boating circles. The simple term covering most of these troubles is corrosion.

VO of the main causes of this corrosion are electrolysis and /anic action. Very often galvanic on is wrongly called electrolysis, us look at each in turn:

Vhat Is Electrolysis? It

cerns chemical changes in metal a solution (saltwater) due to !ow of current (DC battery volt ). It occurs when direct current n a battery or generator passes )ugh the saltwater from one metal t to another. ry this simple experiment: Place copper strips one on each side a glass of saltwater. Connect one the positive and the other to the ative of a battery (6 volts will do).

Jow observe the bubbles at the ative strip and a faint green cloud dng the positive strip. The metal the positive strip is going into solution. If after a short time weigh it with a sensitive scale will notice a loss in weight, ve it long enough and the positive 3 will disappear. This is commonly ed electrolysis. )bviously if you pass battery rent between the metal skin fittings any submerged metal parts, the itive part corrodes away. Stop the r of current and the corrosion >s. ince this passing of current is done intentionally how does it pen? One side of the battery uit to the motor, or to many ine electronic devices—lights, etc. ; connected to frame (earth). If cross-earth any of these circuits, ig a reversed earth polarity gative or positive), by installing

the battery back to front, you immediately set the vessel up for electrolysis.

Remember that the current causing this damage can also flow through the bilge water, as well as the water outside. It will also flow through saturated timbers.

Warning

So be sure all earths are common and don’t ever reverse battery con nections. Negative to frame is obviously preferable. Another way a flow of current is created is by using insufficiently heavy wire to supply devices drawing heavy current.

A voltage drop can occur which gives the same effect as a DC battery voltage, namely, a difference of potential (voltage) between two points underwater.

A good idea is to connect all grounded elements together, inside the boat. This is called earth bond ing. Use very heavy bonding wire, or copper strip as short as possible.

GALVANIC ACTION: Contrary to general opinion, most underwater corrosion has no connection what ever with electrolysis. It is purely a galvanic corrosion. In principle its action is identical with that of the old type primary battery cell.

Two metals are in contact with each other and submerged in salt water. They are of different com position and they are in electrical contact through a metallic path, thus forming a primary battery cell.

Current being generated flows through

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the metallic path, and the met supplying the current corrodes awa So don’t use ordinary cheap bra for underwater fittings, as it is fi of zinc. Some units such as propelle made from a mixture of actr elements will act on themselves in tl same manner. Obviously you mn use good pure seaworthy metals, e.j bronze, monel, etc. (if money we no object, titanium, gold, platinum and you must be sure all underwat fittings are of the same family.

The following is a list of meta and their corrosive effects under sa] water. When any group of the metals is used under saltwater and interconnected or joined, the met closest to the bottom (Base) corrode The metal closest to the top (Nobl< does not corrode or, if it does, does so at a slower rate.

Top

Stainless Steel Monel Bronze Copper Brass Lead Steel-iron Aluminium (pure) Galvanised steel or iron Zinc Magnesium

Bottom

For example, if stainless steel ai magnesium were placed in saltwat (electrolyte) together, the magnesiu would corrode away much before tl stainless steel showed similar effect If zinc and magnesium were placx in saltwater the magnesium wou corrode away just ahead of the zin which wouldn’t be far behind.