BAKER HUGHES - Drilling Fluids Reference Manual

CORROSION
Pressure
Increasing pressure increases the solubility of most corrosive gases. Entrained, or trapped air
rapidly goes into solution in the fluid as the pressure increases, when the fluid is pumped
downhole. This dramatically affects the oxygen content of the fluid, increasing corrosiveness.
pH
Generally, for steel components, corrosion rate decreases as pH increases. At ambient
temperatures, as the pH increases, corrosion rates rapidly decrease. Rates are much slower in
alkaline fluids than in acidic fluids. Little reduction in corrosion rate is obtained as pH is increased
above 10.5 (see Figure 8 - 5).
For aluminum drill pipe and components, corrosion rates increase with increasing; pH. This is due
to the formation of soluble aluminum compounds at high pH. If aluminum components are used in
the drill string, the pH should be maintained in the range of 6 – 9.
Dissolved Salts
The effect of salt concentration is twofold.
1. As salt concentration increases, conductivity rises increasing the corrosion rate (see Figure
8 - 1).
2. Increasing the salt concentration, however, reduces oxygen solubility, (see Figure 8 - 2),
thereby decreasing corrosion rate. The overall effect is a brief rise in corrosion rate due to
conductivity until the salt concentration reaches approximately 18,000 mg/L (Cl – ). Above
this range, as salt concentration increases, oxygen solubility and corrosion rates decrease
(see Figure 8 - 3 and Figure 8 - 4).
CORROSIVE AGENTS AND TREATMENTS
Oxygen (O 2 )
1. Source – Atmosphere. Water additions, solids removal, gas separation equipment and
mixing equipment generally increase oxygen content.
2. Reactions – Oxygen corrosion of Iron (Fe) is cathodic depolarization reaction. Basically,
this is what happens when water and oxygen are present.
BAKER HUGHES DRILLING FLUIDS
REVISION 2006 8-2
REFERENCE MANUAL