General information on Corrosion
Stainless steel is not only used to resist atmospheric corrosion, but it also resists the action of many aggressive chemical substances. This is why it is used in equipment and process pipes in chemical, petrochemical and food plants, among others.
Given the importance of our subject, we will indulge on a brief introduction to this complex phenomenon, stating its causes and consequences.
Whether it regards the design or maintenance of a chemical plant, material selection is a vital issue. The correct choice of materials will mean the difference between normal and economic functioning, and unscheduled, costly halts in production.
It is impossible to eliminate corrosion as a factor, but with the adequate knowledge of its mechanisms, it is possible to reduce its damage and costs to a minimum.
The ways to control its effects are many: modifying the values of the variables involved in the process, material selection, cathodic protection, anodic protection, etc. All these alternatives must be considered carefully.
There is no single material that works aptly in all circumstances. Specialists are continuously developing new alloys (more than 20.000 known to this day), or resorting to other types of non-metallic materials, such as Teflon, PVC, ebonite, etc.
The material selection is the result of a technical and economical analysis of each situation, in which the following factors are taken into account:
| a) Corrosion resistance. |
| b) Mechanical resistance. |
| c) Availability in the market. |
| d) Low cost. |
| e) Workability. |
The reasons which make point a) more relevant than the rest are many.
1. Economic
An unscheduled halt in plant activity, besides the cost of repairs, implies production loss. In today’s enormous facilities, this represents a cost of several hundred thousand dollars. This type of failure may be caused by corrosion (cracks, leaks, etc).)
Product loss due to leakage, besides causing economic loss, may considerably jeopardize the health of the surrounding population by releasing toxic substances.
Rust deposits on pipes reduce the flow’s capacity, with the subsequent decrease in material cargo and increase in energy spending.
Corrosion also entails the consumption of non-renewable resources. The use of zinc for anodic protection reaches several million tons per year.
If the product of corrosion is deposited on heat-transferring surfaces, due to its low thermal conductivity, heat transfer is reduced. This brings about the need to design broader surfaces, which in turn means higher costs.
When calculating pipe dimensions, extra-thickness is considered. This is added to the necessary thickness due to fluid pressure, allowing for the corrosive process.
2. Social
Besides the danger of a toxic substance leak, there is also the possibility of explosions which jeopardize the lives of plant workers.
The former are only a few of the many reasons we could state, but they reflect the vital importance of corrosion in the proper, economic functioning of an industrial plant.
We will attempt to study this phenomenon by answering four questions:
| a) What is it? |
| b) Why is it produced? |
| c) How is it produced? |
| d) How is it controlled? |