In principle, underground corrosion is the electrochemical reaction similar to aqueous corrosion.

Difficulties of soil corrosion problem

The corrosion phenomenon cannot be ¡°visually¡± observed.

Complex process involving various factors.

By experience, P=ktn for steel in soil.

Object of This Study

Prediction of underground corrosion as a function of soil parameters by statistical approach.
- Determination of ¡°k¡± and ¡°n¡±.

System

Polyethylene (PE) coated gas transmission steel (API X65 Gr.E) pipeline

Survey for coating defects and metal loss points by excavation and visual inspection

Experimental Procedure

Broad spectrum for environmental values.
Neutral environment (pH: 4.2 to 8.5).
All corrosion occurred at wet clayey soil.
Iron sulfides and groundwater at corrosion site.
High levels of chloride at corrosion site.
High levels of SRB and APB at corrosion site
Normal distribution of parameters makes it possible to perform statistical analysis.

Classification of variables related to corrosion.

Statistical technique to reduce variables and dimension of problem by coordinate conversion using eigenvector.

Variables in Group I is strongly related to corrosion of steel in soil.

Using these variables, multiple correlation analysis can be made.

Three factors contribute to corrosion

chemical factors (pH, chloride content, pH, resistivity)

microbial factors (resistivity, redox potential, i.e., activity of SRB)

electrical factors (P/S potential, i.e., the effect of CP)

1. A model has been presented to predict the maximum corrosion rate on steel pipes buried in soil environments.

2. The multiple regression analysis with k in P=ktn reflecting the environmental factors and n as a regression
coefficient has been established.

3. The result shows that the predicting equation explained well the field corrosion phenomena.

4. The underground corrosion of carbon steel is affected by chemical and microbial properties of soil such as pH,
resistivity, reduction-oxidation potential, the level of chloride.
It is also important that the effect of cathodic protection should be considered together for the precise evaluation
of the risk of corrosion.