Corrosion Fatigue

Corrosion fatigue is a decrease in fatigue strength due to the effects of corrosion. Corrosion fatigue cracking differs from SCC and hydrogen induced cracking in that the applied stresses are cyclic rather than static. Fatigue cracking is often characterized by “beach marks” or striation patterns which are perpendicular to the crack propagation direction, as shown in Figure 17. Both the stress required for crack initiation and propagation can be lower in corrosive environments. Factors influencing corrosion fatigue include material strength, fracture toughness, and environmental conditions. There are two primary material properties used to assess fatigue, the number of cycles to failure for an applied stress level or the crack growth per cycle for a stress intensity factor.

Stress-Life (S-N) Data

One type of reported fatigue data is stress-life or S-N curves, which plot the stress amplitude versus the number of cycles to failure. This follows the empirical relationship

where,
Δσ = change in stress
σ_f = fatigue strength coefficient
N_f = number of cycles to failure
b = fatigue strength exponent

In a fully reversed, constant stress amplitude fatigue test, Δσ/2 = σ_a , the stress amplitude.

Fatigue Crack Growth Data

Information on fatigue can also be found in the form of crack growth plots. The relation in this case is

where,
a = one half crack length
N = number of cycles
K = stress intensity factor
σ = stress amplitude
C,m = empirical constants

There are three types of fatigue crack growth behavior as depicted in Figure 19. Type A exists for materials affected by the corrosive environment for crack initiation and crack growth. Type B behavior exists for materials where no environmental effect exists below the stress intensity threshold fro SCC. Type C is a combination of types A and B. Aluminum alloys in seawater follow type A behavior as can be seen in Figure 20.

Managing Corrosion Fatigue

Methods to deter corrosion fatigue include the following:

• Employ designs which minimize stresses to the components
• Choose heat treatments that reduce residual stresses
• Use surface treatments that enhance corrosion fatigue resistance such as shot peening or laser treatments
• Use barrier coatings or corrosion preventive compounds to block corrosive species from the metal.