Page 401 - Challenges in Corrosion Costs Causes Consequences and Control(2015)

CORROSION DAMAGE, DEFECTS, AND FAILURES 379

Low-alloy steels are used for wellbore tubulars. Corrosion limits for jointed tubing
2
are set at <0.05 lb/ft of tubing surface area representing a loss of 1/1000 in. in wall
thickness. Pitting deeper than 0.381 mm is not acceptable.
2
The weight loss of 0.02 lb/ft and zero pitting tendency should be met in the case
of low-alloy steel.
2
The guidelines for high-alloy tubular materials are set at 0.05 lb/ft chrome and
duplex steels are used to prevent bottom hole corrosion. Acid corrosion causes sig-
nificant loss in metal along with pitting

0
Fe → Fe 2+ + 2e −
−
+
2H + 2e → H
2
The inhibitors used in acid systems are organic amines, which form an inhibitor
film because of Fe–N interactions. Other inhibitors such as acetylenic alcohols
(propargyl alcohol) may be used.
The mechanism of inhibition by acetylenic alcohols consists of chemisorption of
the alcohol on the metal followed by subsequent polymerization. For maximum inhi-
bition, the hydroxyl group in the alcohol should be in the position to the acetylenic
function, and the acetylenic function should be terminal.
The synergistic blends consist of amines, acetylenic alcohols, surfactants, and oils
to make a readily dispersed product.

Corrosion in Underbalanced Drilling Operations The underbalanced
drilling technique minimizes the harmful effects of drilling fluid invasion by
reducing the hydrostatic pressure of the drilling fluid (32). This method requires
nitrogen, which can be accomplished by using nitrogen membranes. When nitrogen
membranes are used, the impact of injecting 5–8% of oxygen into the drilling fluid
must be considered. The drilling fluids are water-based saline solutions that cause
corrosion of casing, drill pipe, and bottom hole assembly.
Oxygen along with CO or H S causes severe corrosion.
2 2

Fe → Fe 2+ + 2e Anodic reaction
O + 2H O + 4e → 4OH − Cathodic reaction
2 2
Above pH 4, the precipitation of ferric hydroxide can occur. The corrosion because
of oxygen may also result in pitting.
Hydrogen cracking may occur when moist hydrogen sulfide is present and the
steel is high-strength steel under tensile stress. Corrosion inhibitors such as Nowcorr
800 consisting of a film-forming amine and a component that reacts with H S combat
2
sulfide stress cracking (SSC).
Batch treatment of the inhibitor is based on the formula:

Volume of inhibitors = 12 l inhibitor × inch diameter × mile