Answer EKG Question 2

Q2. Discuss the causes of corrosion and means by which corrosion of the following may be limited by manufacturers and ship’s personnel respectively:

A. Internal and External surfaces of auxiliary steam lines;

B. External surfaces of auxiliary boilers;

C. Water boxes of sea water coolers and condensers;

D. Main sea water inlet pipes.

Answer: Corrosion protection of steam line:
The outside surface to be insulated with some light bonded mineral wool padding with stainless steel wire netting, this become the primary barrier. The padding to be covered with some metal jacket fastened and bonded to prevent water entry at joints or where insulation are supported with attachment angles.
Cement coated insulation can be finished with a suitable water proof mastics to prevent water ingress. The type of insulation and method of application should be chosen assure the absence of any shrinkage cracks. wrapping equipment with aluminium foil before applying insulation will reduce the risk of corrosion.
Boiler external surface- The entire exposed area to be covered with lightly bonded mineral wool mattresses of density 150 kg/m3 for surface temperature above 400 deg cel. and with density 100kg/m3 for lesser temperature with S.S netting. For every subsequent layer of mattresses with G.S wire netting. The total thickness could not be less than 3mm. The outer surface should be covered with aluminium metal sheet and circumferential over lapping should be tightly bonded with self tapping metal screws or to be riveted. keen observation during operation to be implemented so that no cover is damaged or detached, causing ingress of moisture which will be soaked by the insulation and permanent source of corrosion will expose.
(a) The internal surfaces of steam lines are affected by air corrosion caused by condensed water remaining inside with air, when the steam is shut off. Corrosion is caused by a warm and moist atmosphere within the pipes. Hence the pipes should be internally coated with a good polymer paint which is coated over the galvanized surface. To maintain this surface in good condition , it is the function of the ship’s staff to ensure that the lines are drained properly when not in use.
The external surfaces are to be coated with a strong heat resistant primer paint and then properly insulated and further protected by metal cladding to protect the insulation. The function of the manufacturer / fabricator is the internal coating and external coating as well as the insulation. When pipes are renewed the ship staff should ensure that the pipes are properly protected in the manner stated above.
(b) The external surfaces of auxiliary boilers are to be coated with at least two coats of good quality heat resistant paint and then covered with an adequate thickness of insulation. The insulation is further protected with steel cladding of at least 3 mm thickness with binding steel straps. This is the job of the manufacturer. The ship staff function is to ensure that during boiler surveys followed by repairs the damaged portions of the insulation are properly restored.
(c) The internal surfaces of water boxes are coated with at least two coats of bitumastic slow drying anti corrosive paint and further fitted with adequate size of zinc slab to prevent galvanic corrosion. This is the job of the manufacturer when the cooler or condenser is new. Subsequently during service when cooler boxes are opened by ship staff for cleaning, they should ensure that the boxes are restored to the same standard as mentioned above.
(d) Main sea water pipes leading from the sea chest to the sea filter and up to the intermediate valve are internally coated with a thick layer of good quality polymer paint by the manufacturer when the ship is new. Subsequently during every dry-docking it is recommended that ship owners follow this procedure if sea water pipes are to last longer.