Oxygen dissolved in water is one of the primary causes of corrosion in the oil field.
When oxygen is present, the most common types of corrosion include pitting corrosion
and uniform corrosion. For more on the theory and mechanisms of these types of corrosion,
go to the theory page.
Oxygen is a strong oxidant and reacts quickly with metal. The maximum amount
of oxygen in water is only 8 ppm, so the mass transport of oxygen is the rate limiting
step in oxygenated non-acidic environments. Controlling the rate of oxygen transport
(often by controlling flow velocity) is thus critical to corrosion control.
O2 corrosion products include iron oxides, including FeO(OH) - goethite,
Fe2O3 - hematite, Fe3O3
- magnetite, and FeO(OH) - ferrous hydroxide.
Corrosion may occur in oilfield applications due to the existence of differential
aeration. In these cases, one section of the metal is exposed to oxygen while the
other is not. The section with no aeration becomes anodic, and is subject to preferential
corrosion. This can occur with pipelines and other metals near the surface. The
first figure shows an example of how a corrosion cell can form when a pipe is buried
below the surface. The soil above the pipe can become aerated due to the digging
and backfilling process, so the top of the pipe is exposed to more oxygen than the
bottom. The bottom becomes anodic and the top becomes cathodic. In the second figure,
a section of pavement restricts oxygen from reaching the pipe in the part of the
pipe under the pavement. That part of the pipe becomes anodic and corrodes preferentially.
Although it is not normally present at depths below around 330 ft (100 m), oxygen
is often introduced in oil production through leaking pump seals, casing and process
vents, open hatches, and open handling. In addition, oxygen removal processes such
as gas stripping and chemical scavenging often fail, allowing oxygen contamination
in waterflood systems.
Oxygen corrosion occurs commonly in drilling fluid, primary production in rod
pumped wells, outdoor rod storage (rusting), oxygen entry into wellbore through
annulus, lower part of well including casing, pump, tubing, lower part of rod string
Prevention / Mitigation
Oxygen removal may be done by mechanical and chemical means. Mechanical means
include gas stripping and vacuum deaeration; chemical means include sodium sulfite,
ammonium bisulfite and sulfur dioxide. Mechanical means of oxygen removal are usually
employed when large quantities of oxygen need to be removed, while chemical means
are used to remove small quantities of oxygen and may be used to remove residual
oxygen after mechanical means have been used.
It is often more economical to exclude oxygen from oilfield equipment than to
remove it after it has entered the system. The most common way of excluding oxygen
is through the use of gas blankets, composed of oxygen free gas such as natural
gas (methane) or nitrogen. Gas blankets may be used on water supply wells and water
storage tanks, supply wells and producing wells, and pumps. Most tanks only require
a few ounces of pressure. The regulator should supply gas at a rate adequate to
maintain pressure when the fluid level drops. Maintenance of valve stems and pump
packing is also important.
To reduce or prevent corrosion in an O2 environment:
Drilling - oxygen scavengers
Producing wells - corrosion inhibitors, oxygen scavengers, elimination of O2
Flowlines - corrosion inhibitors, oxygen scavengers, elimination of O2
- Click on thumbnail to see larger picture
Signs of oxygen corrosion include wide shallow pits and reddish brown rust.
Oxygen corrosion also causes large areas of metal loss on sucker rods.