Corrosion and Fatigue Behavior of High-Strength Steel Treated with a Zn-Alloy Thermo-diffusion Coating
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JMEPEG (2017) 26:5228–5236 DOI: 10.1007/s11665-017-2969-z
Corrosion and Fatigue Behavior of High-Strength Steel Treated with a Zn-Alloy Thermo-diffusion Coating C.P. Mulligan, G.N. Vigilante, and J.J. Cannon (Submitted May 12, 2017; in revised form July 27, 2017; published online September 27, 2017) High and low cycle fatigue tests were conducted on high-strength steel using four-point bending. The materials tested were ASTM A723 steel in the as-machined condition, grit-blasted condition, MIL-DTL-16232 heavy manganese phosphate-coated condition, and ASTM A1059 Zn-alloy thermo-diffusion coated (Zn-TDC). The ASTM A723 steel base material exhibits a yield strength of 1000 MPa. The effects of the surface treatments versus uncoated steel were examined. The fatigue life of the Zn-TDC specimens was generally reduced on ascoated specimens versus uncoated or phosphate-coated specimens. Several mechanisms are examined including the role of compressive residual stress relief with the Zn-TDC process as well as fatigue crack initiation from the hardened Zn-Fe alloy surface layer produced in the gas-metal reaction. Additionally, the effects of corrosion pitting on the fatigue life of coated specimens are explored as the Zn-TDC specimens exhibit significantly improved corrosion resistance over phosphate-coated and oiled specimens. Keywords
coated steel, corrosion, fatigue, surface engineering, thermo-diffusion coating, wear, Zinc coating
1. Introduction High-strength alloy steels remain among the most important engineering materials due to their combination of strength, formability, and ductility along with low cost. However, wear and corrosion issues associated with non-stainless steel based hardware are prevalent across all industries. Some key disadvantages of high-strength steels such as the 4XXX series include poor corrosion resistance and susceptibility to wear. There are a multitude of options in terms of surface modification for corrosion protection including common alternatives such as Cr or Ni plating, galvanizing, sherardizing, phosphating, and painting, or more advanced approaches such as thermal spray or vapor deposition techniques of corrosion-resistant materials (Ref 1, 2). When both improved wear and corrosion resistance are desired, some of these techniques can be combined with heat treatment processes such as nitriding or carburizing or used in a stand-alone fashion such as with hard Cr plating or sherardizing which both provide hard, wear-resistant surfaces. One of the most common low cost approaches to providing some protection to steels is phosphating combined with either a corrosion preventive oil or spray coating such as paint or dry film lubricant. There are numerous components where painting is impractical due to size/shape constraints, the presence of sliding wear, operating temperature constraints, cost constraints, or combinations therein. This leaves a heavy manganese or zinc phosphate with oil such as those called out in MIL-DTL-16232G (Ref 3) as the most common option
C.P. Mulligan, G.N. Vigilante
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