Corrosion Fatigue

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Understanding Corrosion Fatigue Fundamentals

Corrosion fatigue (CF) represents one of the most significant challenges in materials engineering, defined as the sequential stages of metal damage that evolve with accumulated load cycling in aggressive environments compared to inert or benign surroundings. This damage results from the interaction of irreversible cyclic plastic deformation with localized chemical or electrochemical reactions. While environment-enhanced fatigue serves as a modern term, corrosion fatigue remains the traditional designation when emphasizing electrochemical environments.

Fatigue damage occurs when materials experience repeated cyclic or fluctuating loading, even when the stress produced remains below the material yield strength. Various types of fatigue damage exist, including corrosion, thermal mechanical, vibration, and creep fatigue. Determining the active form of fatigue proves crucial because preventive measures differ significantly depending on the specific case.

Corrosion Fatigue in Industrial Applications

Boiler Systems

In boiler tube applications, corrosion fatigue occurs through the combined synergistic actions of cyclic loading and corrosive environments. This discontinuous process involves crack initiation and growth during transient periods. Excessive stresses develop during boiler operation due to restraint at tube attachments and load changes, particularly during cold starts or forced cooling cycles. During shutdown or restart of circulation boilers, thermal stratification of water along the tube length creates additional stress conditions.

Poor water chemistry and its excursions significantly influence both initiation and propagation of corrosion fatigue. The breakdown of the protective magnetite layer represents the key issue, with pH serving as the most decisive chemistry parameter, particularly during low pH excursions.

Automotive Applications

In automobile components where steel faces exposure to aqueous salts from road conditions, corrosion effects become important design considerations. The challenge intensifies as designers demand higher strength and lower weight from components. Heat treatment offers one approach to achieve higher strength without increasing weight; however, as steel undergoes quenching and tempering to increase strength, its corrosion-fatigue resistance decreases. This creates an obvious trade-off between high strength and the material's ability to perform well in corrosive environments, driving automotive engineers to optimize alloy steel strength while maintaining good corrosion resistance.

Mechanisms of Corrosion Fatigue Crack Initiation

Research has identified four primary mechanisms that cause faster crack initiation when steel experiences exposure to corrosive environments:

Corrosion Pits and Stress Concentrations

The stress concentration resulting from corrosion pits represents one of the first mechanisms proposed to explain faster crack initiation observed in corrosion fatigue. These pits create localized stress concentrations that accelerate the crack initiation process.

Preferential Electrochemical Attack

The preferential dissolution mechanism causes decreased corrosion-fatigue initiation life. Fresh metal exposed when slip steps break the specimen surface faces preferential attack by corrosive species. This attack creates stress concentrations in already highly strained areas, causing decreases in material fatigue strength.

Oxide Film Rupture

Oxide film rupture can cause faster crack initiation in certain metals and environments. This effect primarily occurs in materials such as copper, aluminum, and stainless steels where oxide layers form through contact with air.

Surface Energy Reduction (Rebinder Mechanism)

The reduction of alloy surface energy through adsorption of environmental species provides another reason for faster crack initiation. The Rebinder mechanism originally suggested that surface-active agents adsorbed into microcracks present in the material, increasing pressure within cracks to effectively advance crack growth at reduced global stresses. Later modifications to this theory proposed that adsorbing species reduced material surface energy, making it easier to create protruding slip bands on metal surfaces.

Limitations of Current Mechanisms

Each corrosion-fatigue crack initiation mechanism discussed provides good basis for explaining observed effects; however, none can fully explain every case. Corrosion pitting reduces steel corrosion-fatigue life, but similar decreases occur in pitting absence. Preferential dissolution prevails in materials where slip steps protrude from material surfaces, but this mechanism cannot explain effects in materials not prone to this behavior. Oxide film rupture does not apply to most steels where these films are not observed.

The Rebinder mechanism represents the most plausible of the four mechanisms but cannot explain all effects. In steels showing fatigue limits in air environments, the Rebinder theory would predict similar limits in corrosion-fatigue. However, corrosion-fatigue data does not confirm this prediction; rather, corrosion-fatigue strength continues decreasing as life increases. Therefore, while each proposed mechanism explains certain aspects of faster crack initiation in corrosive environments, none can single-handedly explain all involved phenomena.

Corrosion Fatigue Behavior Characteristics

Fatigue has often been described as the number one cause of failure in engineering metals. When corrosive environments combine with fatigue loading, large decreases in metal fatigue life occur. The first notable trend in corrosion-fatigue behavior shows that fatigue limits observed when steel undergoes fatigue in air become significantly lower or completely erased by corrosive environments. Figure 1 illustrates this general trend and demonstrates how metal fatigue strength in corrosive environments continues falling as cycles to failure increase.

Figure 1: Corrosion-fatigue and its general effect on the behavior of steel

Research has found that in most low-alloy steels fatigued in contact with salt solutions, no "safe stress range" exists at which the metal has infinite life.

Pitting Resistance in Stainless Steels

Corrosion fatigue can initiate through pitting corrosion. This may occur on martensitic stainless steels in paper machines but has not been reported for duplex stainless steels. This difference likely results from duplex steels' higher pitting resistance due to increased amounts of chromium, nitrogen, and molybdenum.

The PRE-value (Pitting Resistance Equivalent) calculates according to equation (1) and provides rough estimation of pitting corrosion resistance, with higher PRE-values indicating higher corrosion resistance:

PRE = %Cr + 3.3 × %Mo + 16%N … (1)

Table 1. PRE-values and measured critical pitting temperatures

Grade	PRE	Average CPT [°C]	Measured CPT, min-max [°C]
3RE60 SRG	29	42	37-43
LDX 2101	26	36	24-41
2304 SRG	25	38	36-40
2205 SRG	34	88	86-90