8. Prevention by Design

8.4 Corrosive Environment


 The following are the major ingredients of a corrosive environment:

  1. Temperature

  2. Humidity

  3. Rainfall

  4. Air pollution

  5. Proximity to sea

  6. Dust storms and dust particles

Types of Metals and Alloys

The metal or alloy must have a proven compatibility to the corrosive environment.  For instance, stainless steel 316 with 2% Mo is a better material for seawater service than SS 304 without molybdenum. Brass, bronze, and copper based alloys are highly desirable for salt water transportation, however, they are vulnerable for an environment containing ammonia frequently encountered in agriculture. A good design to prevent corrosion must be compatible with the corrosive environment. Following is a summary of the effect of major contributors to corrosive environment.

Temperature

Temperatures slightly in excess of 50C are observed in several countries, like Kuwait, Saudi Arabia, and the United Arab Emirates. High temperatures in combination with high humidity produce an accelerating effect on corrosion. In a survey conducted in one of the towns in Saudi Arabia, the corrosion-free life of an automobile is only six months. A rapid fall in temperature can cause condensation.

 

Humidity

Corrosion progresses fast when the relative humidity exceeds 75%. Humidity in Europe and British Island often exceed 75%. In certain areas of Ghana, Nigeria, Congo Basin, South America, South-east Asia, and Gulf region, humidity may approach 100% and cause condensation.

 

Rainfall

The rainfall acts as a two-edge sword, excess rainfall washes corrosive materials and removes dirt, debris, and other deposits which may initiate corrosion, whereas scanty rainfall may leave water droplets on the surface and lead to corrosion as salt is present in the air. The frequency of rainfall contributes to humidity.

 

Pollution

In addition to sodium chloride particles in the coastal areas, the atmosphere may contain sulphur dioxide, sulphurous acid, and sulphuric acid which are considered as the worst offender as far as corrosion is concerned. They originates from power stations, refineries, chemical and steel manufacturing plants. The environment is abundantly populated by them in oil producing countries in the Gulf region.

 

Man-made pollution

To above factors, must be added the cumulative effect of man-made pollutants, such as the presence of sodium chloride which are extensively used in deicing of roads in North American and European countries. Use of small amounts can induce high levels of corrosion in road vehicles. In desert regions, the abundance of sand particles accelerates corrosion because of hygroscopic nature of sand particles. The atmosphere may also contain other pollutants, such as carbon monoxide, nitrogen oxides, non-methane hydrocarbons, and methane. Closeness to sea in many tropical areas creates condition highly conducive to the onset of corrosion.

 

Proximity to Sea

Seawater is considered to be equivalent to ⅔% solution of sodium chloride. The salinity of most ocean is 35 per thousand and the conductivity of seawater at 15C is 0.042 ohm/cm. There is abundance of chloride in the marine environment and in industrial zones located in marine environment. Cumulative corrosive effect is caused by both chloride and sulphur dioxide. Chlorides can absorb moisture at relative humidities as low as 40%.