Many thermal fatigue cracks are filled with:

Thermal fatigue occurs when fixed equipment experiences repeated thermal expansion and contraction due to cyclic operating temperatures. High surface temperatures during operation promote oxidation on the metal surface and within open crack paths. As the thermal fatigue cracks initiate and progressively propagate into the material, oxygen continuously reacts with the freshly exposed metal surfaces, causing the crack channel to become filled with oxides. The presence of oxide scale within dagger-shaped, surface-breaking cracks is a primary metallurgical characteristic used in failure analysis to differentiate thermal fatigue from mechanical fatigue, which typically features clean, tight crack faces. Oxide accumulation within the crack tip can also exert wedging action, further accelerating crack growth under cyclic conditions.

A. Chlorides are associated with environmental cracking mechanisms such as chloride stress corrosion cracking (Cl-SCC), not the primary filling material found inside thermal fatigue cracks.
B. Hydroslime is a biological fouling product related to microbiologically influenced corrosion (MIC) and does not form inside thermal fatigue cracks.
D. Sulfides form under high-temperature sulfidation or wet H2S damage mechanisms, whereas thermal fatigue cracks are characteristically oxide-filled due to thermal cycling in air or process fluids.

1. American Petroleum Institute (API), API Recommended Practice 571: Damage Mechanisms Affecting Fixed Equipment in the Refining Industry, 3rd Edition, 2020, Section 3.64.5 ("Appearance or Morphology"), p. 3-311.

2. EPRI, Thermal Fatigue Inspection and Management Guidelines, Electric Power Research Institute Technical Report 1011955, Section 3, 2005.