Heat exchangers onboard ships especially the shell and tube type mostly use seawater as the cooling medium. Seawater is highly corrosive and its continuous flow through the heat exchangers will gradually eat away the metals. Corrosion of the heat exchanger will reduce its lifespan thus the need for protection.
The different types of protection for heat exchangers against corrosion are:
- Use of sacrificial anodes - zinc, aluminium, magnesium, soft iron
- Use of impressed current system
- Use of coating - rubber elastomer, bitumen or epoxy resin
- Use of ferrous sulphate
Use of Coating:
Coating header or water boxes with bitumen, rubber elastomer or epoxy resin will minimize corrosion. This protection approach is commonly used for sea water pipes.
Use of Sacrificial Anodes:
Sacrificial anodes are easily corroded metals used on heat exchangers to protect the headers, tube plates and the tubes from corrosion. Zinc, aluminum, soft iron and magnesium are the most commonly used metals for sacrificial anodes.
Metal alloy with a high negative electrochemical potential are used as Sacrificial Anodes to protect metals with low negative potential. Zinc and aluminum are often used for metal objects in saltwater while magnesium is more suitable for fresh water purposes.
With Soft iron, when eaten away by galvanic corrosion, a fine dust of iron oxide settles on the surface of the metals forming a protective film.
Zinc is usually used as the anode for heat exchangers to protect the tubes, tubes plates and the headers because it is softer than aluminium brass.
The sacrificial anode must be replaced when it completely corrodes otherwise the heat exchanger will no longer be protected.
Principle of sacrificial anode:
Two dissimilar metals connected together and immersed in an electrolyte will form simple galvanic cells and an electric current will flow from one metal to the other through the electrolyte thus forming a complete circuit.
The metal anode or the positive plate from which the current flows will tend to suffer rapid corrosion, called "Galvanic or Electrolytic Corrosion" and the metal cathode or the negative plate to which current flows will tend to be protected from galvanic corrosion.
The direction in which the current flows depend on the composition of the metals or alloys exposed to the electrolyte and also on the hardness and cleanliness of the metal.
Use of Impressed Current System:
Impressed current system is also called Cathodic protection and the parts to be protected are made cathode in respect to another point which is anode.
The electric current generated by the ship is converted into low voltage direct current and impressed on the anode. Inert anodes made of LEAD SILVER with PLATINUM bi-electrodes are employed, capable of passing current into the water without deterioration.
The major difference between cathodic protection system and sacrificial anode method is that electric current is not generated by it but impressed upon it thus the anode is not eaten away or corroded.
This system was first introduced for protection of the ship's hull, now commonly employed for protecting non-ferrous components in salt water systems such as the heat exchangers.
It's very important to impress the right amount of current onto the anodes because too little will set up the natural corrosion cells and if it's too much, then the paint and protective coatings will be damaged as well as waste of electrical energy.
Use of Ferrous Sulphate Treatment:
The presence of iron compounds or iron corrosive products in seawater system is said to have beneficial effects on the heat exchanger tube etc..
Adding ferrous sulphate solution in the salt water system can help reduce impingement attack. The solution is injected into the salt water system at a point close to the main sea water inlet to the ship. The standard type of injection system consists of a polypropylene mixing tank and PVC piping, and the solution is injected by means of gravity feed.