Stress-Corrosion Cracking and Corrosion Fatigue Impact of IZ-C17+Zinc-Nickel on 4340 Steel

New protective material could replace cadmium and aluminum coatings on critical components.

The protection of cathodic metallic materials used for aircraft components, like 4340, Aermet 100, and PH 13-8 corrosion-resistant steel, is critical to keeping the steel from pitting and cracking due to exposure to the operating environment. Two important properties are resistance to stress-corrosion cracking (SCC) and corrosion fatigue. These are insidious failure mechanisms that can lead to part failure in service.

T-45 pivot with IVD aluminum coating, passivated with a chromate conversion coating

Cadmium and aluminum coatings are currently used to protect high-strength steels from corrosion, pitting, and cracking. These coatings are applied on new components and also at Navy Fleet Readiness Centers (FRC) during component repair and overhaul. Both are effective but each has shortcomings.

Cadmium is electroplated or sometimes applied in a vacuum chamber. The electroplating process allows for coating of all component surfaces using a low-cost method. However, cadmium is toxic and carcinogenic and alternatives are desired to eliminate these risks.

Aluminum is electroplated or applied by the ion-vapor deposition (IVD) process. At Navy FRCs, the IVD process is used. This requires a vacuum and is limited by line-of-sight, so not all surfaces of components can be coated depending on their geometry. IVD is a relatively high-cost practice and requires more maintenance than a cadmium electroplating line.

Alternatives to cadmium have been investigated for at least 50 years, with IVD aluminum being an early commercialized alternative. More recently, zinc-nickel alloys have been optimized to have coating properties that are very close to cadmium. The deposition process for these new alloys is electroplating.

A company called Dipsol provides a commercial zinc-nickel plating solution, IZ-C17+, that has been optimized for use on high-strength steels to perform similarly to cadmium and aluminum. One shortcoming of the data available is the ability of the coating to minimize SCC and corrosion fatigue of the substrate material and how it compares to cadmium and aluminum. These are two critical requirements for sacrificial coatings used on Navy and Marine Corps aircraft components.

Prior work documents the method to assess SCC and corrosion fatigue and the performance of electroplated cadmium on 4340 steel by itself, with a MIL-PRF-23377 Class C primer, and with both the primer and a MIL-PRF-85285 gloss white topcoat. This protective coating system is typical for high-strength steel components.

The purpose of this research was to assess the ability of electroplated IZ-C17+ zinc-nickel to suppress SCC and corrosion fatigue of 4340 steel using methods previously developed by the Materials Engineering Division. The zinc-nickel coating was assessed by itself, primer only, and with the primer and standard gloss white topcoat used on fleet aircraft components. The primer used was a standard MIL-PRF-23377, TY I chromate-based primer currently used on high-strength steel parts that are coated with cadmium or aluminum.

This work was done by Craig Matzdorf, Charles Lei, and Matt Stanley for the Naval Air Warfare Center. NAWC-0003

This Brief includes a Technical Support Package (TSP).
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Stress-Corrosion Cracking and Corrosion Fatigue Impact of IZ-C17+Zinc-Nickel on 4340 Steel

(reference NAWC-0003) is currently available for download from the TSP library.

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