Ship piping system is the connection of the main and vice machine and related equipments, seawater piping play an important role of cooling equipments, fire fighting, ballast water treatment and others, which is extremely important in ship piping system.

Metal pipes are  widely used, and they will face corrosion problem because its transmission medium is seawater and seawater is highly corrosive. The direct economic loss is quite a lot, as the global annual steel equipment scrapping because of corrosion is equivalent to 10 percent of annual output. What’s worse, the indirect economic loss caused by corrosion of the equipment is more incalculable.

On the ship, the corrosion of the piping and the equipment damage caused by corrosion cause inconvenience, affect the ship’s underway rate, and even affect safety of the ship’s personnel and the whole ship. What’s worse, the corrosion of the seawater piping system is more serious. Usually,perforation corrosion will occur in coolers and other pipelines, and a large amount of seawater will permeate through the cabin, which seriously affects the safety of ship navigation.

A failure analysis by BRITOILJ, a British Marine engineering company, shows that 33% of all equipment failures are caused by corrosion. The design and construction of ships have been continuously studied and adopted in various ways to mitigate or avoid corrosion.

Function of the seawater piping system and the current situation of corrosion

2. 1 Function of seawater piping system

2. 1. 1 cooling equipment

Air cooler, condenser, lubricating oil and fresh water cooling of main and vice engine, lubricating oil cooling of stern shaft (submarine door, sea water pump, cooler)

2. 1. 2 ballast

Because of little changes in carrying capacity, We insufflate fresh water into the special ballast tanks for before going to sea, and most of cargo ships useseawater ballast.

2. 1. 3 marine fire protection

Seawater is a natural extinguishing agent.

2. 1. 4 sanitary flushing

such as the toilet water on board.

2. 2 Corrosion status of seawater piping system.

2. 2. 1 The seawater piping at the upper part of the cabin is corroded more seriously than which at the bottom of the cabin.

2. 2. 2 The seawater pipeline of smaller diameter is corroded more seriously than which of larger seawater pipeline.

2. 2. 3 The pipeline which is closer to the pump outlet is corrodedmore seriously.

2. 2. 4 The water pipeline near the elbow and flange interface is corroded more seriously.

2. 2. 5 Corrosion is often honeycombed.

3 Causes of corrosion of seawater and several factors that affect corrosion.

Causes of corrosion of seawater and
several factors that affect corrosion.

3. 1. Causes of corrosion of seawater piping.

3. 1. 1 contact corrosion

Two metals or alloys of different potential contact with each other (direct contact or through wire connection).When in the same electrolyte solution, the metal with low potential is constantly corroded, and the potential difference between the two metals is larger, corrosion will be more seriously.

This is known as macro battery corrosion.

3. 1.2 intergranular corrosion

It is selective corrosion on the grain boundary of metal or alloy. The different metallographic phases  and crystal defects of metal materials are in different electrode potential, and they will produce microbattery in electrolyte solution.3. 1. 3 stray current corrosion.

One of the electrolytic corrosion which is the uncontrolled flow of current from the outside of the field. Mostly occurs in places where it’s easy to generate static electricity.

3. 1. 4 impact corrosion

It refers to the electrochemical corrosion on the metal surface and the damage of fluid impact.

. It is easy to occur in silty water area .

3. 1. 5 cavitation corrosion

It refers to the high speed of fluid flow, turbulent flow, and bubbles occur in the negative pressure are compressed and burst under high inner pressure. Besides, bubbles continuously impact the surrounding metal, and work together with other corrosion to damage pipeline. It is integrated corrosion types which is mainly of cavitation corrosion, supplemented by other corrosion.