Corrosion in metals

Galvanic Action

Corrosion within cooling systems can occur if the coolant, i.e. water, has not been properly treated. The corrosion can take the form of acid attack with resultant loss of metal from a large area of the exposed surface, or by Oxygen attack characterised by pitting. A primary motive force for this corrosion is Galvanic action

The Galvanic Series.

Or Electromotive series for metals

The metals closer to the anodic end of the list corrode with preference to the metals towards the cathode end.

A galvanic cell can occur within an apparently Homogeneous material due to several processes on of which is differential aeration where one area is exposed to more oxygen than another. The area with less oxygen becomes anodic and will corrode.

Galvanic action within metal

Galvanic action due to temperature gradient

This situation can exist in cooling water systems with complex layout of heat exchangers and passage ways within the diesel engine. Systems containing readily corrodible metals such as zinc, tin and lead alloys can complicate and intensify problems by causing deposit formations.

Differential Aeration

-Where only a single metal exists within a system corrosion can still take place if the oxygen content of the electrolyte is not homogenous. Such a situation can occur readily in a jacket water system as regions of stagnant flow soon have the oxygen level reduced by the oxidation of local metal. The metal adjacent to water with reduced levels of oxygen become anodic to metals with higher oxygen content electrolyte in contact with it. Generally, the anodic metal is small in comparison the cathode i.e. the area of stagnant flow is small compared to the area of normal flow of electrolyte, and high rates of corrosion can exist. One clear case of this is the generation of deep pits below rust scabs.

Corrosion of Metals

Steel (Fe)

Will readily corrode by the reaction with oxygen in the water primarily by galvanic action.

The Iron reduces to Iron ions at anode, the oxygen is reduced by combining with water and electrons passed from the anode (by iron changing to ions) to hydroxyl ions. Temperature, pH and the concentration of oxygen all affect the rate of corrosion.

The oxygen reacts with the Fe²⁺ to form Hematite (Fe₂O₃) . This is a reddish brown loose deposit. With reduced oxygen content the formation of Magnetite (Fe₃ O₄) will occur. This is a more tenacious layer and forms a protective boundary on the metal preventing further corrosion. This layer may be removed in low pH or high pH conditions.

Scabs and tubercles of Ferric oxides and Ferric hydroxides form over an active pit.

Stainless Steels

These are alloys of steel with high chromium content (around 11%). The alloying process results in a material with excellent corrosion resistance. Oxygen combines with the chromium and iron to form a tenacious self healing oxide layer.

The disadvantage of stainless steel is that in low oxygen environments, such as boil feed, the corrosion resistance is actually reduced. In addition stress corrosion cracking and pitting can occur when in the presence of chlorine ions. In this way stainless steel is not recommended in situations were stagnant sea water might exists at it could perforate quicker than mild steel. The chlorine ions are the correct right size to enter the atomic matrix of the metal and their concentration accelerates corrosion by the propogation of cracks. Catastrophic failure can occur.

Copper(Cu) and Copper alloys

Used in heat exchangers due to there high heat conductivity. Copper corrosion in oxygenated water is slow due to the time taken for oxygen to diffuse throught the oxide layer.

As copper is a relatively soft metal water velocities must be kept low. Its presence can lead to heavy pitting if deposited in steel systems. Ammonia in the water can remove the oxide layer and promote rapid corrosion

Aluminium (Al)

Is essentially inert in neutral water up to about 180'C. It is ampoteric meaning it will corrode rapidly in high and low pH conditions. In the presence of Sodium Carbonate or sodium hydroxide at pH above 9 this corrosion is particularly severe. These conditions may exist were boiling occurs concentrating hydroxyl ions.

Zinc (Zn)

Is anodic to steel and is often used as a protective coating on steel in a process called galvanising. It is ampoteric, corrosion increases in high and low pH conditions.

Note! At temperatures above 60'C the anodic/cathodic relationship with steel is reversed. This is of particular importance in engine cooling systems. The author has joined a vessel were the third engineer was merrily fitting anodes to the cylinder heads of a daihatsu generator engine. The fact that the engine was fresh water cooled with inhibitor treatment also was ignored

If galvanised pipes are to be fitted in a cooling circuit the coating must first be removed by controlled acid washing.

Factors affecting corrosion rates

Temperature

As a rule of thumb for each 10'C rise in temperature doubles the rate of corrosion.

The rate of oxygen diffusion increases in an open system with temperature up to around 80'C. A rapid tailing off then occurs due to the solubility of oxygen. For this reason open system feed tanks seen on many vessels have heating coils which maintain the temeprature at 85'C or higher.. In a closed system there is no such tail off as the oxygen cannot escape

pH/Alkalinity

The electrochemical nature of the metal will determine its corrosion rate with respect to pH. The corrosion rate of iron reduces as the pH increases to about 13 due to the reduced solubility of the Fe ions. Aluminium and zinc, being ampoteric, have rates of corrosion that increases with pH higher or lower than neutral.