BAKER HUGHES - Drilling Fluids Reference Manual

CORROSION
CORROSION
Chapter 8
Corrosion is the destruction of a metal by chemical or electrochemical reaction with its
environment.
INTRODUCTION
Corrosion is a severe and costly problem in the drilling industry. Because the tubular goods are
mostly iron and many drilling fluids are water-based, corrosion is inevitable. Four conditions must
be met, however, before corrosion can occur.
1. An anode and a cathode must exist.
2. The anode and cathode must be immersed in a conductive medium.
3. A potential difference between the anode and cathode exists.
4. There must be a coupling to complete the electrical circuit.
The anode and cathode exist on the drillpipe itself. The drilling fluid may serve as the electrolytic
medium, and the coupling is created by the drillpipe steel. The potential difference is due to the
crystalline structure and different metals used in the drilling pipe alloy.
FACTORS AFFECTING CORROSION RATE
Several factors affect the rate at which corrosion proceeds. Most of these factors are interrelated
and have a compound effect on the corrosion rate. The basic relationships are as follows:
Temperature
Two different effects occur.
1. As temperature increases, the corrosion rate increases. If all other factors remain constant,
the corrosion rate doubles for each 55°F (13°C) increase in temperature.
2. Increasing temperature decreases the solubility of corrosive gases (O 2 , CO 2 , H 2 S), thus
decreasing the corrosiveness of the fluid due to these gases. Note that “solubility” here
refers to the solubility of gas at surface pressure, and does not include chemical reactions
of gases such as CO 2 and H 2 S with the fluid.
BAKER HUGHES DRILLING FLUIDS
REVISION 2006 8-1
REFERENCE MANUAL