Fatigue behavior of Zn–Ni and Cd coated AISI 4340 steel with scribed damage in saltwater environment

• Characterization of corrosion fatigue behavior of 4340 steel coated with Cd and Zn–Ni.
• Both coatings provide equal extended protection from corrosion fatigue relative to uncoated case.
• A scribed damage in both coatings did not degrade corrosion fatigue behavior for both Cd and Zn–Ni.
• Hydrogen re-embrittlement is not an issue with electroplated Zn–Ni without and with damage.
• Environmentally friendly Zn–Ni coating is capable to replace unfriendly Cd for high strength steel.

High strength steel (AISI 4340), coated with electroplated cadmium and zinc–nickel, was characterized for tension–compression fatigue behavior without any damage and with a scribed damage in coating under the salt water environment (3.5 wt.% NaCl) at room temperature. Both coatings provided an equal amount of extended protection relative to uncoated specimen against the corrosion fatigue. Additionally, the scribed damage in both coatings had no detrimental effect, such as hydrogen re-embrittlement, on the corrosion fatigue. These observations were same when specimens were tested at either 10 Hz or 1 Hz. However, corrosion fatigue behavior of Cd and Zn–Ni coated HSS was a combination of cycle- and time-dependent phenomena. Damage mechanisms were similar in both coatings without any damage or with a scribed damage. Damage initiated as a typical fatigue involving transgranular fracture, then a transition region consisting of combination of intergranular and transgranular fractures, thereafter intergranular fracture, and finally failure due to overload. The 1 Hz specimens had relatively larger transition region than 10 Hz specimens.