The Principals of Seawater Corrosion

Seawater corrosion develops due to two different reactions: Electrolysis and Galvanic corrosion, these are often confused with each other, but are destinctly different. Electrolysis is caused when an external current, a stray current, finds a pathway across two metals in the presence of an electrolyte, salt water. The two metals can be the same type or different it doesn't matter, though the end result can be very different.

Galvanic corrosion is when two dissimilar metals are in contact with each other in the presence of salt water.

The destruction of metallic components on a boat is a permanent problem demanding constant attention. Even a basic understanding of the processes can go a long way to minimising the damage.

What is Salt Corrosion?

Metallic corrosion or 'Oxidation' is the process of either stealing or donating electrons, when the electrons are stolen or donated, the metal atoms will dissolve in the water. Salt water is a much better conductor that fresh, so the rate of corrosion increases dramatically. Total salt removal is the only way to stop this process as even the smallest amount of residual salt will keep the reaction going.

The effects of oxidation on metals such as mild steel are obvious and the destruction is rapid. Metals such as stainless steel, titanium, manganese bronze and anodised aluminium minimise this destruction by forming a thin, protective oxide layer covering their surface. This oxide surface is self-healing when scratched, thereby minimising the surface degradation.

Aluminium Corrosion is Different

At first Aluminium corrodes very quickly, much faster than Iron or Steel, but once an oxidised surface has developed, it bonds very tightly providing a protective surface, this is the process of anodising to protect aluminium.

When salt is present, the chloride (Cl-) ions attack this oxidised layer, penetrating and exposing fresh aluminium. This then corrodes only to be stripped off again in a never ending cycle. So the natural protection Aluminium gets from an oxide coating is useless against salt! Marine grade Aluminium provides considerably more resistance to this corrosive onslaught.

The style of corrosion illustrated is often referred to a pitting corrosion. It is where the oxide surface breaks down, developing crevice where the corrosion can persist. Different metals vary in their susceptability to pitting corrosion as illustrated in the chart.

Pitting corrosion or crevice is often encountered in threads and between surfaces where dirt can build up creating a galvanic reaction. The use of zinc based greases on maring threads will significantly reduce this type of corrosion.

The correct installation of Zinc anode blocks is also important and will significantly reduce pitting corrosion.

The impact of fast flowing water should not be overlooked. Aluminium is highly susceptable to increased corrosion in fast flowing water as is steel and zinc. This needs to be considered with motor cooling systems where water flow in pumps can be very high and are often quick to deteriorate in salt water cooled inboards.

Galvanic Corrosion

Galvanic corrosion happens when two dissimilar metals are in contact in the presence of an electrolyte; salt water. Consider boat fittings installed with stainless fixings. These are a prime candidate for galvanic corrosion. Any water which contains impurities will act as an electrolyte. The table of metals listed on the left will highlight the problem boat owners have.

As a rule, the further apart the metals are on the table, the more they will react in the presence of an electrolyte. Stainless Steel and aluminium are at opposing ends, yet 316 stainless is the most commonly used material for fixing aluminium fittings.

The risk of galvanic corrosion is reduced by limiting the variety of materials used during boat construction and by ensuring sacrificial anodes are installed. This is particularly important on Aluminium boats.

Electrolytic Corrosion

Electrolytic and galvanic corrosion are often confused with each other. Electrolysis is caused when an external current, called a stray current finds a path between two metals in the presence of an electrolyte. The two metals may be of exactly the same type or different types

If two different metals are exposed to a stray current, the metal at the top of the chart on the left may not necessarily be protected, as it is with galvanic corrosion. If the stray current is sufficient to overcome the natural galvanic current then the more noble metal may well become the anode and be destroyed. The rate of electrolysis attack is dependent on the amount of current present. A stray current caused by a short circuit can ‘eat’ metal components away very quickly.

The risk of electrolytic attack can be minimized by making sure all electrical systems, are installed correctly and well maintained. Installing and using a battery isolater can also reduce the chance of stray current.  

Anything that is exposed to salt water, salt air or ground salts will be a candidate for salt corrosion. Even when the salt is dry it is still a problem since salt is hydroscopic, attracting moisture. Any available salt will allow the corrosion to continue, often unseen until it is too late and the damage is done.

Because of salt's hydroscopic property, removing salt residues is not an option if your equipment is to survive long term, it has to be thorough. The ability of SX50 and Buster 50 to neutralize salt, makes reducing seawater corrosion easy. Once the salt is neutralised, corrosion is halted!

Prevention is Cheaper than Repair and Replacement

Some Salt Removal Methods can be Costly!

A customer decided not to use SX50 because his mechanic's suggested a "better method"....

"...The recommendation was 3:1 water and hydrochloric acid left in the system for 3/4 hr then flushed out. This seemed to work very effectively although I only left it for 1/4 hr before flushing the system..."

We decided to get some expert advice from an Industrial Chemist who specialises in chemical/metal corrosion.

"...The use of uninhibited hydrochloric acid (HCl) on ANY metal surface would not be recommended. Any HCl purchased "off the shelf" would not have an inhibitor system added to it - this means that the acid will not stop acting when it reaches "clean metal" but will continue to etch into the metal surface.

Effectively, uninhibited hydrochloric acid continues to etch into the metal surface at the same rate that it removes scale or rust. If you have a motor which has varying depths of scale - as most do - this means that the acid will be eating through scale in some parts of the motor, but will be eating through the base metal where the scale is thin.

Further, HCl will cause pitting of the surface as it will follow a corrosion pathway in areas of the metal that may be heat stressed - possibly leading to breakthrough. Further, residual chloride ion will continue to promote corrosion on ferrous parts even after the acid has been flushed out.

Finally, HCl will "de-zinc" brass and bronze leading to "sponginess" in the metal and loss of mechanical strength in yellow metal components.

SX50, on the other hand, has a number of advantages:

1. It's a "soft" acid - effective at removing scale, but weak at attacking base metals.
2. It has a "three way" inhibitor system - with specific inhibitors for ferrous metals, aluminum & "yellow metals". This means that it will quickly etch to the metal surface and then stop reacting once it hits base metal.
Further, these inhibitors, in combination with the film forming protective agents present in SX50, protect the metal from ongoing corrosion.
3. Being chloride free, SX50 will not cause "chloride corrosion" as seen with HCl - in fact the exact opposite is true – SX50 displaces chloride ion at the metal surface and protects the metal against salt attack.
4. Finally, the film forming agents within SX50 have a lubricating effect on seals and bushes...."

Bruce Heath, BAS App Chem, Grad Dip Tech Mgt MBA MRACI-CChem