Corrosion Protection

Corrosion Protection

Unsurpassed Corrosion Protection

Commercial and military testing has proven that AlumiPlate® aluminum is the highest performing protective coating for engineered components ^(1).

Pure aluminum (Al) is benign and fully compliant with Restriction of Hazardous Substances (RoHS) and Registration, Evaluation, Authorisation and Restriction of Chemicals (REACH). Electroplated Al benefits from the synergistic effect of multiple mechanisms to offer unsurpassed corrosion protection.

AlumiPlate’s unique thin, dense, high purity electrodeposited aluminum plating has been tested and qualified by the US Department of Defense (DoD) as the highest performance corrosion protection available. It is used in many aerospace and ground vehicle programs as the coating of choice for the most severe corrosion problems.

Cd and ZnNi vs AlumiPlate® Al (ASTM G-85 SO₂ F-35 Lightning II program)
“NAVAIR Public Release 08-172: Cadmium Alternative Coating Corrosion Performance on Steel”

Environmentally Friendly and 100% RoHS and REACH Compliant

Electroplated aluminum is the highest performing, environmentally-friendly coating fully compliant with European RoHS, REACH and WEEE regulations and also US initiatives for minimization of hazardous materials use.

US DoD testing has been primarily driven by increasing regulations to find an alternative to hazardous cadmium plating with hexavalent chromate, which is prevalent in US weapons systems as a corrosion protection coating. Cadmium plating is a toxic and antiquated process still widely used throughout the aerospace industry. In fact, the largest domestic user of toxic “cadmium and yellow hex chrome” is the US Government.

Several executive orders have tried to address the issue and mandated finding and migrating to benign replacements. Additionally, European directives on Restriction of Hazardous Substances (RoHS); Registration, Evaluation, Authorisation and Restriction of Chemicals (REACH); and Waste Electrical and Electronic Equipment Device (WEEE) have targeted cadmium and hexavalent chromate.

These recently implemented regulations have a global impact and have accelerated the testing and adoption of non-hazardous coating alternatives, mostly based on zinc, nickel and tin (or as a combined alloy coating such as zinc-nickel).

AlumiPlate’s 100% REACH / RoHS compliant thin dense high purity aluminum coating is the highest performing replacement technology for cadmium coatings. Only electroplated aluminum can claim superior performance to cadmium in a variety of accelerated corrosion tests:

ASTM B117 neutral salt spray — 2000+ hours

ASTM G85 Appendix 4 SO₂ (sulfur dioxide) salt spray — 500+ hours

ASTM D610 Rating after Marine Exposure Test — No corrosion signs after 6 months (0% corroded area)

Mechanisms That Protect Against Corrosion

AlumiPlate aluminum provides excellent corrosion protection particularly in salt and industrial acidic environments. Multiple mechanisms are at work and result in outstanding corrosion protection.

• The AlumiPlate Al plating is typically more than 99.99% pure, lacking impurities and contaminants that typically act as corrosion initiation sites. It has excellent inherent resistance to chlorides, sulfides, nitrides and fluorides.
• The AlumiPlate Al plating is dense, pore free and provides a physical barrier to corrosion.
• The AlumiPlate Al plating is self-healing in oxygen environments. When breached, it immediately reforms a thin, tenacious, chemically inert, non-degenerating and protective oxide.
• The AlumiPlate Al plating sacrificially protects most metals due to its high electronegativity. In the presence of a corroding electrolyte (such as salt water), electroplated Al will preferentially corrode, protecting substrates such steels and even other Al alloys.
• The AlumiPlate Al plating can be used at temperatures up to 450 °C and higher. Other coatings cannot even be considered for high temperature exposure because of their lower melting temperatures.

To learn more about the anti-corrosion features of electroplated Al, please refer to this brief technical bulletin.

Hydrogen Embrittlement, Environmentally-Assisted Cracking and Stress Corrosion Cracking

Pure electroplated Al is the coating of choice for critical high strength and non-redundant steel and titanium components. High strength landing gear, pins, couplings, fasteners and other flight-safety critical components are protected from hydrogen embrittlement and from corrosion by the AlumiPlate Al coating.

Aluminum plating is a unique non-aqueous solvent-based process and is inherently less likely to cause hydrogen embrittlement (HE) than other coatings. Water-based coatings such as cadmium, zinc, nickel, tin, zinc-nickel, and tin-zinc produce copious amounts of hydrogen which can be absorbed by the substrate and can lead to hydrogen embrittlement. This embrittlement mechanism is often called process-induced hydrogen embrittlement and requires a 23 hour, 375 °F heat treatment or “hydrogen embrittlement (HE) bake out”. Al plated components do not require this bake since the plating process is aprotic and free of any hydrogen.

Hydrogen embrittlement can also occur in the field. In severe environments, corrosion of the substrate and the protective coatings forms hydrogen. High strength components are especially sensitive to hydrogen pick-up and the resulting environmentally induced hydrogen embrittlement. Failure related to this embrittlement mechanism is called environmentally-assisted cracking (EAC). When the component is under load, the failure is known as stress corrosion cracking (SCC).

Electroplated Al is the only coating that has been shown to effectively manage and mitigate environmentally-induced as well as process-induced HE. The US DoD and their suppliers have carefully evaluated and verified hydrogen embrittlement test results. The AlumiPlate Al coating has been tested and compared with a wide variety of coatings, showing superior performance in process-induced hydrogen embrittlement, and environmentally-induced hydrogen embrittlement (SCC and EAC).

Galvanic and Dissimilar Metal Corrosion

Because of its lower weight, the use of high strength aluminum alloys as structural materials has progressed from aerospace to the automotive and other industries. New military and commercial aerospace and ground vehicles using aluminum require a solution to galvanic corrosion.

Electroplated Al protects from galvanic corrosion in steel-to-aluminum (and other dissimilar metals-to-aluminum) applications. For example, in applications where steel fasteners must be used in conjunction with aluminum structures, the use of aluminum electroplated fasteners protects those aluminum structures from galvanic corrosion better than any other plated surface.

Copper, steel, stainless steel, high temperature materials (iron/nickel/chrome/cobalt superalloys) and titanium can be electroplated with AlumiPlate Al to stop galvanic corrosion with Al structures. Galvanic corrosion tests have also shown that electroplated aluminum outperforms Cd, Zn, Zn/Ni and Sn/Zn for minimizing galvanic corrosion with magnesium components.

High Temperature Corrosion and Thermal Cycling

Components exposed to high temperatures or large thermal cycles can benefit from AlumiPlate Al.

Electroplated Al has a high temperature envelope. It is routinely used in applications up to 450 °C (or even higher where the exposure is intermittent).

Aerospace high strength steel components are sensitive to liquid metal embrittlement of zinc coatings and solid metal embrittlement of cadmium coatings. Severe service conditions can create elevated temperatures at which zinc or cadmium become mobile and are absorbed by the substrate. Electroplated Al (660 °C) has a much higher melting temperature than zinc (420 °C) or cadmium (321 °C) thus minimizing liquid and solid metal embrittlement concerns.

AlumiPlate Al is a semiconductor industry preferred anti-corrosion coating for protection against hot fluorine gas, typically used in the cleaning cycle of the manufacturing process in semiconductor wafer chambers. The combination of resistance to fluorine, high temperature capability, high purity and compatibility with aluminum structures is unique to electroplated aluminum. Semiconductor OEM’s have replaced costly high temperature capable FeNi alloys (Haynes and Hastelloy) with less expensive Al plated stainless steels, while experiencing the same or better corrosion protection and contamination control.

The AlumiPlate Al coating can survive severe thermal cycling due to its high adhesion strength, low internal stresses and very high ductility. The plating has even been successfully used in applications with thermal cycling from room to cryogenic temperatures.

Outstanding Corrosion Protection Results

AlumiPlate Al has a long history of superior performance on a variety of accelerated corrosion tests by aerospace, automotive, chemical processing, marine and other industry-leading OEMs. No other coating can claim the combined benefits and features of electroplated Al (please refer to the table below for a comparative analysis versus other coatings).

Until recently, ASTM B117 neutral salt fog was the most common comparative test to gage corrosion performance. Electroplated Al has demonstrated extreme protection to salt fog, with results as high as 6000 hours for a 12.5 µm (0.0005”) thick coating.

More recently, new aerospace and ground programs are using the more stringent ASTM G85 Appendix 4 SO₂ (sulfur dioxide) salt spray test. For example, Lockheed Martin’s F-35 II Lightning program requires 336 hour resistance for its most severely exposed components. Corrosion from salt water and diesel fumes in ships and aircraft carriers represents one of the toughest and most severe environments requiring the best possible protective coating.

Electroplated Al not only outperformed all other coatings in cyclic sulfur dioxide testing, it was the only coating to meet the requirement with exposures as high as 500 hours. For this reason, Lockheed Martin selected electroplated Al to protect exposed electrical connectors in wheel wells.

Kesternich testing also measures resistance to sulfur dioxide. Because sulfur dioxide is a common acid rain constituent, SO₂ resistance is an indicator of performance in locations where acid rain is possible. AlumiPlate aluminum survives an outstanding 20 cycles of the 2.0 g/l concentration and 40 cycles of the 0.2 g/l concentration testing.

AlumiPlate Al has been well tested by aerospace OEMs and their suppliers for corrosive fluid performance. In all cases, AlumiPlate Al outperformed other alternative coatings. This is particularly true when tested with runway de-icing fluids that contain potassium formate (HCO₂K). Potassium formate has been reported to accelerate corrosion of galvanically protected steels, notably cadmium plated landing gear components.

Contact us to discuss how your components can be protected from severe corrosion through the use of Electroplated High Purity AlumiPlate® Aluminum.