Boeing approved Cadmium alternatives
Boeing and engineering specialist ES3 has tested, evaluated and certified Zinc-Nickel plating as a viable and cost effective alternative to Cadmium. Read more about its performance compared to Cadmium and how one of the USA’s largest air force bases is already using it on their plating line.
As the aerospace sector continues to move in favor of Cadmium alternatives, it can be reassuring to learn that some of the industry’s leading voices are already trialing, evaluating and certifying innovative plating solutions. In many cases these perform the same, or even better than Cadmium when it comes to factors such as hydrogen embrittlement, thickness, adhesion and corrosion.
In 2009, Boeing’s Steve Gaydos accompanied Craig Pessetto and Jay Randolph, from aerospace component engineering specialist ES3, in delivering a presentation at the ASETSDefense: Sustainable Surface Engineering for Aerospace and Defense Workshop.
Their presentation focused on Zinc-Nickel (Zi-Ni) plating as a viable, cost effective alternative to Cadmium and offered a number of comprehensive case studies and reports highlighting proven successes within the industry.
The tank plating line at Utah’s Hill Air Force Base, one of the USA’s major air force bases, was one of the case-in-points mentioned, along with Boeing’s evaluation of Zi-Ni on threaded fasters and components.
There are a number of interesting findings highlighted throughout this presentation, including:
- Dispol, a popular tank plating solution using IZ-C17+ Alkaline Zinc Nickel plating, passes all certification tests including hydrogen embrittlement to ASTMF-519, thickness and adhesion and corrosion to ASTM B117
- It has been proven that the tank IZ-C17 Zinc Nickel plating provides excellent coverage and performs as well or better than Cadmium in all qualification tests
While the main topic of discussion during this presentation was the specification of Zi-Ni as a viable alternative to Cadmium, the presenters also touched on a number of other alternative processes, including sputter and cold spray Aluminum – both of which are approved for use by Boeing.
You can download the full ASETSDefense presentation here. If you would like to discuss your Cadmium replacement requirements in more detail, call our dedicated team, or read more about our world-renowned Zi-Ni solution.
Cadmium alternatives for coating high-strength steel
The USA Department of Defense conducted a comprehensive research project into Cadmium alternatives for high-strength steel applications. LHE Zinc-Nickel performed as well or better than Cadmium in almost all the accelerated tests. Read more about its performance when compared to other Cadmium alternative alloys.
When considering which Cadmium alternative to specify for the coating of high-strength steel landing gears and component parts, it can be helpful to review unbiased research into which coatings perform the best under scrupulous test bed conditions.
One such information source is the report into phase I and II of a comprehensive research program conducted by the Joint Cadmium Alternatives Team, using resources provided by the USA Department of Defense’s Environmental Technology Demonstration and Validation Program.
This study is cited as “overarching and objective simultaneous evaluation of the most promising Cadmium alternative coatings for high-strength steel applications.”
In it they tested the following alloys against controlled Cadmium and IDV-Al:
- Sputtered and electroplated Aluminum (Ai)
- LHE Zinc-Nickel (LHE Zi-Ni)
- Acid Zinc-Nickel (Acid Zi-Ni)
- Electroplated Tin-Zinc (Sn-Ni)
Interestingly, at the time of this report, no single alternative had been authorized to replace cadmium applications, so in a market which at the time was neutral and open to all alternative coatings solutions, the results really speak for themselves.
LHE Zn-Ni performed well in almost all of the tests, with laboratory technicians identifying it as having performed “about the same, or better than Cadmium in all but one of the accelerated corrosion tests, with H-RE and fatigue remaining the principal performance concerns especially for Navy applications.”
Unfortunately, this testing program ended before the third phase began which means insufficient data was generated to fuel an implementation plan. That said, it was widely agreed that there was little need to further investigation the viability of sputtered and electroplated Al as an appropriate alternative to Cadmium.
Using the data gathered during phase I and II of this trial, the following actions ensued:
- NAVAIR prepared a limited authorization letter for AlumiPlate
- It was suggested that LHE Zi-Ni should replace sputtered aluminum pending engineering approval
- Other industry figureheads began further assessing the favorable adhesion and corrosion resistance properties of Zi-Ni
You can download the full ESTCP report here. If you would like to discuss your Cadmium replacement requirements in more detail, call our dedicated team, or read more about our world-renowned Zi-Ni solution.
The world’s first study into Cadmium alternate finishes
Boeing commissioned an independent research report which, for the first time, directly compared all viable Cadmium alternate finishes, including Zinc-Nickel, and the brush plate repair process. Read the executive summary here and download the full report.
With a growing global desire from the military sector to move away from the use of carcinogenic Cadmium, and an increased requirement for all processing and maintenance activities to comply with OSHA standards and EPA regulations, Boeing performed independent research into the comprehensive evaluation of environmentally-compliant alternatives to electrodeposited Cadmium for low-strength steel applications.
This research was conducted by Boeing – St Louis, which was tasked by the Interorganizational Work Authorization (IWA) to conduct a number of very specific tests in order to assist Boeing – Seattle in their ongoing investigations.
Using electrodeposited Cadmium as the control finish, Boeing – St Louis compared; Alkaline Zinc-Nickel, Acid Zinc-Nickel, Tin-Zinc and IVD Aluminum as possible alternatives. Their study was centered around three key tasks:
- Prepare test specimens with alkaline Zi-Ni and with IVD Aluminum
- Test selective specimens with acid Zi-Ni, alkaline Zi-Ni and Tin-Zinc platings or IVD aluminum coating
- Control specimens with Cadmium to qualify the Cadmium replacement finishes
At the time of this research, these alloys were already being used within the defense and aviation markets to some extent, but this was the first study ever commissioned to directly compare them as Cadmium alternative finishes.
Taking an admirable hands-on approach to finding the right Cadmium alternatives for the defense sector, Boeing – St Louis produced two extensive test matrices of specimens as a result of their laboratory testing. Their research included strippability of alkaline Zi-Ni and IVD Aluminum, as well as scribed and unscribed corrosion resistance tests of all prepared specimens.
As part of this in-depth evaluation, the team were also asked to evaluate the brush plating repair process, which included corrosion resistance, appearance, adhesion, thickness and paint adhesion tests.
This project demonstrated that alkaline Zi-Ni and IVD Aluminum coatings can be successfully applied to a wide variety of specimen configurations, and are therefore considered acceptable Cadmium replacement finishes.
Since Boeing’s independent research in St Louis, SIFCO 4018 has become the only recommended brush plating alternative for Cadmium.
Electrodeposition of Zinc and Zinc Alloys
The fifth edition of ‘Modern Electroplating’ includes a chapter dedicated to the research behind Zinc and Zinc-alloys. Download the relevant chapter to learn more about the development of non-toxic, cyanide-free Zinc Electrolytes and the performance benefits of Zinc-Nickel in the ongoing replacement of Cadmium coatings.
Now in its fifth edition, ‘Modern Electroplating’ is considered the definitive resource for electroplating.
Compiled by over 30 global experts, the book is revised in line with industry advances to provide an ongoing, and undisputed, point of reference for electroplating professionals.
In the latest edition, there is a chapter providing detailed information on the researched behind the composition of Zinc and Zinc-alloys – affording the reader insight into the application method and the evolution of the Zinc or substrate material.
The new Modern Electroplating explores the idea that Hydrogen embrittlement is of grave-concern for high-strength, low-alloyed steel. It also explains that the substrate is mainly affected during the first deposit layer and that once full coverage has been achieved, Zinc and Zinc-alloys constitute a good diffusion barrier against Hydrogen embrittlement.
This newly added information is particularly pertinent for those considering Zinc-alloys as a potential replacement for Cadmium coatings within their industry, providing a number of important considerations and factors, such as:
- An enforcement in environmental constraints has aided in the development of non-toxic, cyanide-free Zinc electrolytes
- These Zinc deposits may encounter some contamination of up to 0.5% of Carbon, originating from the additives
- The corrosion protection of steel increases with an improved nickel content of up to 15% – beyond that, the deposit loses its sacrificial properties
- Zinc-Nickel alloys were found to show the best corrosion resistance
ES3 recommends LHE Zinc-Nickel for aerospace
Following a Technical Interchange Meeting at the Aeronautical Systems Center, a letter of recommendation from aerospace component engineering specialist, ES3, was made public. This letter and its accompanying engineering report recommends the use of low hydrogen embrittlement Zinc-Nickel as a direct replacement of Cadmium for high-strength steel landing gears and components.
The report explains in great length a series of tests based on the expansive use of Zi-Ni within the automotive industry as an alternative to Cadmium coatings. It also details how the alloy would need to be reformulated to create a Low Hydrogen Embrittlement alternative before being used within the aerospace and defense sectors.
Having detailed their findings at in great length, the report suggests the imminent progression of phase III, which would focus on the implementation of LHE Zi-Ni as a recommended alternative to Cadmium.
This enlightening report describes how machine-scribed (corroded) Zi-Ni panels tested in accordance with ASTM B 117 pass with no white corrosion for 96 hours. We also learn that the age of the PVC tank liners is an important consideration due to the risk of corrosion and solution contamination.
The issue of fatigue was also highlighted in research conducted by the Joint Cadmium Alternatives Team, using resources provided by the USA Department of Defense’s Environmental Technology Demonstration and Validation Program (ESTCP).
Fatigue data in relation to Zi-Ni as a suitable alternative to Cadmium has been widely evaluated, with Goodrich Landing Gear, Boeing Commercial HBN Inc. and Heroux-Devtek all concluding that is it acceptable to use Zi-Ni on high-strength steel landing gear components.
An overview of Cadmium and its alternatives
Steve Gaydos’ report on Cadmium and the testing of alternative coatings provides an insightful overview of why Cadmium became the material of choice and how Zinc-Nickel and Aluminum platings are measuring up in the performance stakes. Read an overview and download the full report.
If Cadmium replacement is new to your agenda, or has recently jumped to the top of your to-do list, downloading ‘Comments on Cadmium Alternatives and Testing’ is a great place to start.
This report, written by Steve Gaydos from Boeing Research and Technology, provides a detailed and insightful overview of Cadmium, why we need to replace it and which alternatives are most suitable.
Why is this report different? Because it takes a step back from the replacement of Cadmium to provide readers with a foundation knowledge of the offending alloy, its performance credentials, why it was so successful and ultimately why we need to stop using it.
By better understanding this background information, specifiers, engineers and technicians can more clearly appreciate why LHE Zinc-Nickel and Aluminum are deemed the most suitable alternatives.
This report affords us the opportunity to take a broader perspective on the issue of Cadmium replacement, and contains many important performance criterion and application statistics.
- Cadmium cannot be used next to Titanium due to the risk of Titanium embrittlement
- Aluminum can be used in fuel tanks and next to Titanium, but while this is a non-embrittling and environmentally friendly alternative, it is considered a ‘line of sight’ solution and cannot be use on internal diameters
- Zi-Ni is a direct ‘drop in’ replacement process for Cadmium and has minimal impact to the supplier base
US Army Research Lab investigates Zinc alloys
The US Army and its professional researching bodies have been a credible source of information in the hunt for Cadmium alternative alloys for the aerospace and defense sectors. In this report, Zinc-Nickel-Tin and Zinc-Iron are evaluated for use within in the US military. Read more and download the full report.
The Army Research Laboratory (ARL) is part of the US Army Development and Engineering Command, and a leading source of reputable Cadmium replacement research and evaluation.
Tasked with discovering the best technologies for US soldiers, specifically when it comes to weapons systems, they began testing Cadmium replacement coatings back in 2001.
Recognizing the importance of finding a suitable replacement for Cadmium within the aerospace market and throughout multiple military sectors, the ARL’s crucial research was to identify coatings which could be included in the “Cadmium Replacements for Defense Systems” program.
This particular report focusses on two Zinc alloy coatings; Zinc-Nickel-Tin and Zinc-Iron electroplating. It was the first time these coatings had been so rigorously tested.
Using AISI 4130 steel panels, tests included:
- Current potential
- Salt fog testing
- Appearance (as received)
- Coating thickness
The report concluded that both coatings were acceptable with respect to the cursory testing performed within the context of their examination for both adhesion and salt fog resistance. But further tests were identified as vital before either alloy can be officially recommended for use by the US aerospace defense sectors.
The next stage of testing would be designed to assess these alloys for functional performance in a fastener application. The suggested tests were:
- Torque-tension testing
- Gallery testing
- Paint adhesion testing
- Operational chemical compatibility testing
There were a number of key findings within this research. It discovered that at high temperatures (>150°F), Zinc alloy coatings become noble rather than sacrificial, leading to steel corrosion. While Zinc-Nickel-Tin shows better current potential, which is a closer match to the published values for Cadmium electroplating.
A non-embrittling Tin-Zinc brush coating solution
As experts in applied surface concepts, SIFCO conducted its own research into Cadmium alternative brush plating solutions. Understanding that this carcinogenic coating has been all but eradicated in the automotive industry by Zinc alloys, SIFCO developed another non-embrittling alternative suitable for the aerospace market.
As a precursor to the development of its new Tin-Zinc brush plating system, SIFCO reviewed and evaluated the performance of both alkaline and neutral Tin-Zinc baths. This research discovered that although the hardness and corrosion resistance of the alkaline bath were acceptable, the elevated solution temperature required for the deposition of the Tin-Zinc was highly detrimental to the anode cover material that is a requirement for brush plating. Most importantly, the plated deposit did not pass hydrogen embrittlement testing.
The neutral Tin-Zinc baths performed similarly. Although more easily applied at room temperature and with acceptable corrosion protection, the deposits failed to pass hydrogen embrittlement tests – even after a relief bake.
The product development, to which this research was crucial, has resulted in SIFCO bringing to market a neutral, sulfate-glucinate, Tin-Zinc solution (SIFCO Process® Tin-Zinc LHE Code 4019) which provides a deposit of 80% Tin and 20% Zinc, and contains no toxic chemicals.
Tests with type 2a HE rings and type 1a.1 HE notch bars have shown that when deposited at > 50°C (>122°C), the deposit meets the ASTM 519 requirement without a post-plating relief bake. If plated at room temperature, a neutral Nickel pre-plate should be used.
This research confirmed that Tin-Zinc alloys can provide a high-performance sacrificial corrosion protection coating for steel by combining the barrier protection of Tin and the galvanic protection of Zinc, without the bulky corrosion product associated with simple Zinc coating. This level of protection is especially critical when considering salt fog testing, with the white corrosion appearing within 24 hours in the absence of a conversion coating, and becoming extensive after just 96 hours of exposure.
Download the full report [here], or learn more about SIFCO’s Cadmium replacement solutions [here].
Nanostructured Zinc based electrodeposits
The successful electrodeposition of nanostructured Zinc alloys is an exciting discovery in the search for a Cadmium alternative coating for fasteners in weapons systems. This research by SERDP highlights the potential performance outputs and cost savings offered by this non-toxic anti-corrosion method.
When considering which is the right Cadmium alternative for your application, it is important to read research from several different authorities on a variety of different potential solutions.
Much of the research we have seen into Cadmium replacement for the aerospace and defense sectors has focused on low hydrogen embrittlement brushed or tank plated Zinc-alloys, but this research, which is split across two reports, provides something altogether different.
The Strategic Environmental Research Development Program (SERDP) is part of the Department of Defense’s environmental science and technology initiative. Their unbiased research is different for three reasons:
- It focuses on alternative coating methods never before researched
- It tests the concept of a multi-layer coating system
- It is application-specific, looking for a solution for the protection of fasteners in weapon systems
Focusing on nanostructured alloys which consist of relatively smaller grain sizes than those of existing metal deposits, the SERDP team used tank plating to deposit the alloy, while exploring the use of pulse electrodeposition with nanostructured zinc-based alloys.
The tests conducted to formulate this report were:
- Corrosion resistance (using salt spray)
- Coating thickness (must not exceed 20 microns)
- Torque/tension (GMW3359, Ford WZ101, 110607)
- Hydrogen embrittlement (ASTM F519)
It was discovered that fine grain structures produce significant benefits over conventional direct current plating when using pulse electrodeposition processes. It has therefore been concluded that pulse electrodeposition is capable of producing a coating which delivers similar or superior performance levels to traditional Cadmium plating.
Some other key considerations include:
- Due to the spherical structure of Zinc dust not providing enough surface contact for sacrificial activity to occur, conventional Zinc coatings are deemed inadequate for plating applications
- Electroplated deposits are self-limiting because the coating film will insulate and redirect the deposit to the bare areas – this process is called ‘throw power’ and is one of the largest advantages of electroplating over spray, powder, and dip applications
In its conclusion, the SERDP research team recommends a multi-layer coating of Zinc Phosphate, a Zinc flake primer and an electrocoated top coat. Why? Because Zinc flake primers are superior when compared to Zinc rich primers for corrosion resistance and adhesion.
Importantly, pulse electrodeposition of nanostructured Zinc alloys can be implemented within the existing Cadmium plating line for tank, rack and bulk plating, which could translate to considerable time and cost benefits to many manufacturers and OEMs.
University thesis provides outside view on Cadmium replacement
Reading an unbiased opinion can often help you make the right choice for your application. This PhD thesis explores Aluminum-based alternatives to Cadmium, looking at the importance of combatting Hydrogen embrittlement and other important considerations. Download it here.
When considering which Cadmium replacement is right for your application, seeking the opinion of someone outside of your industry can provide an unbiased view of each presenting solution.
In 2008, Cranfield University Applied Sciences student, Emanuele Cardilli, wrote a PhD thesis on Aluminium based coatings for Cadmium replacement, highlighting some important factors for consideration.
As an outsider to the aerospace sector, Cardilli evaluates all previous research in the report to provide a balanced view of findings to date, before applying his own ideas to the issue.
This thesis acknowledges the importance of combatting Hydrogen embrittlement and re-embrittlement, while highlighting the role Aluminum-based coatings (SermeTel and Alcotec) could play in the eradication of Cadmium as a sacrificial protective coating on high-strength steels.
Cardilli notes a number of interesting considerations in the thesis, including:
- Aluminum-based coatings cause less embrittlement than Cadmium
- Bronze bushes and bearings increase the corrosion rate of Aluminum if left uncoated
- Hydrogen re-embrittlement can occur when a sacrificial coating corrodes