The Destruction of Metal by Chemical or Electrochemical Action. A problem that is not always apparent! Not So

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1 CORROSION The Destruction of Metal by Chemical or Electrochemical Action Technical Information Sheet No.1995/40/FG All too often, vessels can be tied up undergoing expensive repairs, or at worst, vessels and lives are lost due to flooding... the cause CORROSION of pipes and fittings. A problem that is not always apparent! Not So With a basic understanding of corrosion this problem could have been eliminated the day the offending pipes and fittings were installed. This data sheet aims to impart some basic information about corrosion, particularly the type that has been brought to light due to recent flooding of fishing vessels, and the associated loss of life. The type of corrosion seen as the most likely cause of engine room flooding, is known as Dissimilar Metal Corrosion. Also, called Bimetallic or more correctly Galvanic corrosion. This is the result when two different metals are coupled to form, what is known as, a Basic Wet Corrosion Cell.

2 What is a Wet Corrosion Cell? Four essential components make up a Basic Wet Corrosion Cell 1. The Anode 2. The Cathode 3. An Electrolyte 4. An Electrical Connection The instant all four components of the Basic Wet Corrosion Cell are in place, corrosion of that section of the installation will begin, the most common areas being in sea water cooling and heat exchange systems. Example "How can it be controlled?" Remove one of the four components of the cell. A short length of mild steel pipe, which is connected from the sea water suction valve, of brass construction, which in turn is connected to the cupronickel seawater cooling circuits of the main propulsion engine. The area of the mild steel is much less than the area of the couple. Therefore, an immediate reaction is initiated and aggressive corrosion of the mild steel commences. Furthermore the velocity of the water is high and scouring of the pipe at the seacock bend commences and breaks down the protective oxides of the inner surfaces of the mild steel pipe. Failure of the pipe is swift and occurs without warning. The seawater quickly engulfs the engine space, by which time the shut-off valves are no longer accessible, the pumps are out of action and flooding occurs. This is not a hypothetical scenario, it is real, and happens all too frequently. -2-

3 Remedy The mild steel pipe is flange mounted to the brass shut-off valve. The flange bolts are of steel. Insulators were placed between the flange plates and inside the bolt holes of the flanges, and front and back of the securing nuts and bolts. Question The corrosion ceased, Why? AnswerBecause one component of the basic wet corrosion cell has been removed The Electrical Connection. Care must be taken when initiating remedies such as insulators, to prevent inadvertent coupling of the pipes either side of the flange and thus reactivating the corrosion cell. For example - pipe mounting brackets. How do I know what materials are compatible with the materials of the main engine? Consult with the engine manufacturer. He can advise on the most suitable material specification for connection to the engine and thus cause the least threat to the integrity of the system. The engine manufacturer will have selected a material specification to provide guaranteed protection for the engine. A simple rule of thumb developed in America, and still in use today, classified metals and alloys into four groups - 1. Magnesium and its alloys. 2. Cadmium, zinc, aluminium and their alloys. 3. Iron and steel, lead, tin and their alloys (other than stainless steel). 4. Copper, chromium, nickel, titanium and their alloys. Stainless steel and graphite. Combinations are only regarded as incompatible if the two metals come from different groups. -3-

4 Dissimilar Metal Combinations to Avoid Bronze propeller on a steel shaft Aluminium fair waters fastened to a steel hull Steel bolts in bronze plates Bronze unions and elbows, used with galvanised pipe Bronze sea-cocks on iron drainpipes Brass bilge pumps on boats with steel frames Brass, bronze or copper fasteners in steel frames Stainless steel pennants on steel mooring chains Bronze or brass rudder posts, with steel rudders Bronze rudders with steel stopper-chains Steel skegs (rudder shoes) fastened with bronze or brass leg screws Steel and brass parts in the same pump Insulation Where conditions are such that dissimilar metals cannot be avoided, certain precautions can be taken to lessen the galvanic effects. The most important point to remember is, wherever practicable, all metals should be insulated from each other either by using plastic/ceramic inserts or sleeves. Relative Area Take into account the area of the two metals. It is possible that large areas of mild steel may be cathodic to smaller areas of more noble metals, and that corrosion of the more nobel metal occurs. If failures occur, do not replace like with like or it will certainly occur again. Look for the reason of the failure, e.g. Insulated flange couplings are breached by bridging insulated areas, thus reactivating the wet corrosion cell. When choosing any material for the engine-room installation, particularly for the seawater system, the effect on the system as a whole should be considered. No part should be considered in isolation. -4-

5 Seawater Cooling System - Component Materials The table below shows components used in typical seawater cooling systems. It should be considered whether the option of a low initial outlay would prove, due to repeated maintenance and consequent vessel down time, to be more expensive in the long-term, than a system with low maintenance costs. Table 1 - Components used in a typical Seawater Cooling System Component Low first cost/high maintenance cost system High first cost/low maintenance cost system Pipe Galvanised steel * 90/10 cupronickel Flanges Steel Cast or forged 90/10 cupronickel. Steel weld overlayed with cupronickel. Gunmetal. Water box Cast iron or fabricated steel 90/10 cupronickel with external steel reinforcement. Fabricated steel lined with 90/10 cupronickel. Ni-resist cast iron (for smaller sizes). Tube plates 60/40 Brass/naval brass Nickel aluminium bronze. 90/10 cupronickel. Tubes Aluminium brass 70/30 cupronickel (particularly 2% Fe+2%Mn). 90/10 cupronickel. Pump casing Cast iron or leaded gunmental Cast cupronickel. Nickel aluminium bronze. Admiralty gunmetal. Ni-resist Type D2. Pump impeller Gunmetal Monel alloy 400 or 500. Stainless steel (CF3 and CF8). Nickel aluminium bronze. Pump Shaft 119 Naval brass Monel alloy 400 or 500. Stainless steel, Type 316. Nickel aluminium bronze. * It should be noted when choosing galvanised steel for the pipework of the seawater cooling system, galvanising should only take place after fabrication, otherwise the welding of flanges and pipes will destroy the galvanised coating. Also attention should be paid to ensure that burrs and sharpe edges, following welding, are ground down prior to galvanising. -5-

6 References UHLIG. (1948) Corrosion Handbook. Galvanic Corrosion. P SHRIER, LL. (1963) Bimetallic Corrosion in Aqueous Solutions. P1.73. BUTLER, G., and ISON, H.C.K. (1966) Corrosion and its prevention in waters. Bi-metallic Contacts. TODD, B. (1975). Selection of materials for high-reliability seawater handling systems. Nickel Development Institute. Courtesy of Inco Limited. TRETHEWEY, K.R., and CHAMBERLAIN, J. (1988) Dissimilar Metal Corrosion. P Acknowledgements MAN B & W Diesel A/S Mermaid Marine Engines Cummins Diesel Finning Cat Mitsubishi Diesel Engines Kelvin Diesels Ltd Perkins Engines Ltd KRUPP Mak Ltd For further information contact - The Sea Fish Industry Authority Technology Division St Andrews Dock, HULL HU3 4QE Telephone (01482) Fax (01482)