Corrosion in Drilling Pipe

What is the main difference between corrosion fatigue fracture and stress corrosion fracture of oil drill pipe?

1. Crack Initiation Regions
Stress corrosion cracking: Can initiate on smooth surfaces, provided the material is subjected to a specific corrosive medium (e.g., carbon steel in H₂S) and a constant tensile stress exceeding a threshold.

Corrosion fatigue cracking: Cracks almost always initiate in areas of surface stress concentration, such as corrosion pits, mechanical damage, or geometric notches. Alternating stress significantly reduces the nominal stress required for crack initiation.

2. Crack Propagation Dynamics
Corrosion fatigue cracking: The subcritical crack propagation rate is mainly governed by the stress intensity factor range and significantly affected by the load cycle frequency (lower frequency, longer corrosion duration, faster propagation).

Stress corrosion cracking: Crack propagation is primarily time-controlled. When the stress intensity factor exceeds the threshold for the material in a specific environment, the crack propagates rapidly at a relatively constant rate, extremely sensitive to the duration of the static load.

3. Macroscopic and Microscopic Morphology
Stress corrosion cracking: The fracture surface is macroscopically rough, and crack bifurcation is common. Microscopically, corrosion-induced brittle fractures are mostly intergranular or cleavage-related, exhibiting patterns resembling "rock candy" or "river patterns," with the fracture surface often covered by corrosion products.

Corrosion fatigue fracture: Macroscopically, the fracture surface shows fatigue-like "beach streaks." Microscopically, although corrosion may obscure typical fatigue striations, fatigue banding characteristics are still observable, and it typically lacks the purely intergranular or cleavage morphology of stress corrosion.

Scenery where oil drill pipe is most prone to fracture:

a. Corrosion fatigue fracture

When the drill pipe rotates at high speed in a curved section of the well.

Fracturing mechanism: Corrosion pits form stress concentration points → crack initiation and propagation under cyclic stress. Corrosion accelerates the fatigue process.
Characteristics: The fracture risk mainly depends on the number of stress cycles and the corrosion rate; the development speed is relatively slow.

b. Stress corrosion fracture

When the drill pipe is in a static tensile state (e.g., stuck pipe, suspended) and in contact with H₂S-containing formation fluid.

Fracturing mechanism: Under tensile stress, a specific medium triggers an electrochemical-mechanical synergistic reaction on the material surface, leading to brittle cracking. 

Characteristics: Once conditions are met, cracks propagate extremely rapidly, often resulting in sudden, unpredictable fractures.

Preventive Measures:

Corrosion Fatigue Fracture Prevention Methods:
1. Control wellbore trajectory to reduce alternating stress.
2. Use internally coated drill pipe.
3. Optimize drilling fluid properties (deoxygenation, corrosion inhibition).

Stress Corrosion Fracture Prevention Methods:
1. Strictly prohibit non-sulfur-resistant materials (such as P110 steel) from contacting H₂S.
2. Use sulfur-resistant steel-grade drill pipe (such as S135G, S135S).
3. Control drilling fluid pH (alkaline environments can inhibit corrosion).

The drill pipe is an essential part of the drill string. Its main function is to transmit torque and transport drilling fluid, and the wellbore is deepened by the gradual lengthening of the drill pipe. Therefore, the drill pipe plays an important role in oil drilling.

Read more: Casing Pipe vs. Drill Pipe or What is Drill Pipe Made of?