Simulating high-level corrosion
New requirements for military-grade galvanic corrosion protection require improved corrosion simulation tools. We hear more from CTO Keith Legg, Senior Scientist Siva Palani, and CEO Alan Rose at Corrdesa.
Half a century ago, the US Department of Defense (DoD) issued the MIL-STD-889 standard, defining the requirements for avoiding galvanic corrosion in military equipment. Ever since, the requirement for avoiding galvanic corrosion in military aircraft, vehicles, ships and infrastructure has been based on galvanic tables derived from data originally developed for US Navy ships.
The work of the US Office of Naval Research Sea-Based Aviation (ONR SBA) programme has confirmed and validated what materials engineers have long known – that galvanic corrosion severity cannot be determined from the galvanic series. As a result, the US Naval Air Warfare Center Aircraft Division (NAWCAD) is updating MIL-STD-889 to base it correctly on the galvanic current between materials, not the galvanic potential difference.
This means that determining galvanic compatibility of interfaces in structures will no longer be a matter of looking up generic alloy potentials in galvanic tables, but will require recourse to reliable electrochemical data and computational methods to determine galvanic currents between specific alloys and coatings, with different heat treatments and surface treatments.
Here, we will discuss the significance of these changes and what they mean for the use of design rules and materials choices in future equipment. Further, we look at what new findings in corrosion modelling teach us about better ways of protecting galvanic interfaces in an era when use of composites is increasing, while ever-stricter environmental regulations are forbidding the use of tried and true protective coatings on which designers have always relied.
A military background
The US Navy and Marine Corps comprise the world’s second-largest air force, with a total of over 3,700 aircraft of all types. The Naval Air Systems Command (NAVAIR) provides full life-cycle support for naval aircraft, weapons and systems operated by sailors and marines. NAVAIR covers the spectrum from R&D, test, evaluation to training, repair and in-service engineering with logistics support. Key objectives are optimum readiness and affordability by reducing operating and sustainment costs.
Corrosion is one of the biggest degraders of readiness in naval aviation, resulting in aircraft not being mission-ready for approximately 30 days per year. Corrosion of US Navy and Marine Corps aircraft costs approximately $3.6bln per year and accounts for almost one third of all maintenance costs – a NACE International study estimated that 25-30% of corrosion issues could be avoided if optimum corrosion management practices were followed.
Evidence from aircraft teardowns shows that 80% of structural damage initiates from corrosion pits, and we know that in air structures the most common corrosion is galvanic. Corrosion problems are compounded by the loss of our best anticorrosion materials because of environmental concerns.
Clearly, having a better understanding of current galvanic corrosion together with usable predictive tools and methodologies based on the correct science should contribute significantly to reducing corrosion costs. But to be effective the correct science must be incorporated into specifications and design rules that are used in the design phase and followed when making repair decisions. There are many examples where standards have been correctly applied, but have still resulted in poor design choices or oversights, such as a corrosion issue with F-22 fighters which resulted in $228m in repair costs. ‘The root cause of this problem lay within the galvanic couple between the conductive gap filler and aluminum skin panels,’ said DoD Corrosion Policy and Oversight Office, Director Daniel J Dunmire in 2011.
Current practice and specifications
Typically, the impact of galvanic corrosion is assessed on the basis of a galvanic series, using galvanic potentials – open circuit potentials – obtained for individual metals under what’s called uncoupled condition. Guidance documents, such as the ASM Metals Handbook 13A, MIL-HDBK-729, 1568, and the UK DEF-STAN 00-970, as well as specifications such as MIL-STD-889C, MIL-STD-1250, MIL-DTL-14072E, ASTM G82-98, are all based on such galvanic tables, and suggest that corrosion severity depends on galvanic potential difference (∆E).
MIL-DTL-14072E proposes that a ∆E less than 250mV (millivolts) implies galvanic compatibility. In reality, galvanic potential only tells us whether galvanic corrosion is likely, but tells us nothing about the corrosion rate, which depends on surface electrochemical reaction kinetics.
In order to capture the kinetics, we need the polarisation behaviour of each material involved, as explained in the following sections. MIL-STD-889 has already started to recognise the shortfall of the present method. Appendix B of the latest release, MIL-STD-889C, states that the maximum corrosion current is actually identified by the crossing point of the material polarisation curves. However, release C does not include polarisation curve data or model materials. This shortfall will begin to be rectified in the next release.
New practice proposed
NAVAIR Senior Materials Engineer, Victor Rodriguez-Santiago is leading an update of MIL-STD-889. He summarised the problems with the present MIL-STD-889, and outlined the new approach of using galvanic current, not potential, to determine galvanic compatibility, and including modern materials and coatings.
The standard will change in several phases, with the first planned for implementation in 2019. Beginning with a table of galvanic currents between different materials, the standard will change in the next year or so to a process based on computational methods. For this approach to work we must have accurate data, consistently acquired. Rodriguez-Santiago presented results of a round robin of same-batch samples and a standard protocol, developed by the ONR SBA team. Following the protocol, several commercial, defence and academic laboratories measured the polarisation data and returned results to NAVAIR. Data taken by laboratories that accurately followed the protocol were consistent enough to justify the use of the protocol and hence the revised MIL-STD-889.
Note that the curve-crossing methodology will be proposed as a means of calculating the corrosion current between connected, dissimilar materials. The advantage of this is that it provides a consistent and quantitative metric, calculating an actual corrosion rate, although the same polarisation data can be used for 3D computer-aided engineering (CAE) models.
Because the polarisation curve is the result of a set of simultaneous anodic and cathodic processes, it can be deconvoluted into its constituent reactions, making it possible to determine self-corrosion rate – and hence galvanic acceleration factor, i.e. galvanic corrosion rate/self-corrosion rate – and to derive the polarisation curve as a function of electrolyte layer thickness.
In response to the upcoming revision to MIL-STD-889D, under funding from the Office of Naval Research Sea-Based Aviation programme, Corrdesa has already developed Corrosion Djinn, a fast and easy-to-use, online, software tool. It is based on the curve-crossing approach, with a consistent electrochemical database gathered using the NAVAIR protocol. It can be used to optimise material choices by easily comparing options for alloys, finishes and treatments. It has been verified and validated per MIL-STD-3022 and is currently being used by NAVAIR.
Corrosion around fastener holes is a common maintenance problem, for which the typical repair method is to insert a stainless steel bushing. Using Djinn, it takes just a few minutes to set up three possible couples to examine the impact of repair options – anodising the Al 7075-T6 airframe, inserting a 15-5 PH stainless steel bushing, or using a Ti6Al4V bushing. All the relevant polarisation curves are pulled from the database and crossed to calculate the respective corrosion currents.
Of the repair options, some are clearly poor choices. For instance, while a good way to protect aluminum from self-corrosion, anodising does little to protect aluminum in a galvanic couple. In fact, it often makes corrosion worse by creating a few deep corrosion pits very close to the galvanic interface.
And mentioned already, galvanic tables would tell us that the best combination is 15-5 PH/Bare Al. Corrosion rate, however, tells us that Ti6Al4V/anodised Al is the best. It also tells us that the best way to combat galvanic corrosion is not by protecting the aluminum, but by modifying the bushing to minimise the oxygen reduction reaction. While the curve-crossing approach of Corrosion Djinn is a fast and accurate way to analyse simple systems, full 3D CAE analysis is required for complex assemblies, time-dependent analysis and lifing.
Use of galvanic tables for predicting galvanic corrosion is almost universal. Most defence contracts require suppliers to use MIL-STD-889, or equivalent, to minimise corrosion, and manufacturers of non-military equipment follow the same approach. The specification shift from galvanic tables to accurate galvanic corrosion rates will reduce corrosion damage and structural failure, especially in complex systems such as electronics and aircraft. Tools already exist to meet the new standard, and new or improved tools will be developed to meet the growing need.
Successful adoption of the new approach means it must be built into design software and written into design rules. Fortunately, since CAD has replaced drafting tables it is a relatively straightforward matter to incorporate a simple computation into the design workflow. However, it depends on a database of polarisation curves that must be available to any design or corrosion analysis software.
This must be a large, validated database of electrochemical data, acquired in a well-defined, consistent manner. Such a database is being assembled by NAVAIR and Corrdesa under the ONR SBA programme and is built into Corrosion Djinn. Building, maintaining and continuously updating such a database will require a long-term commitment from both the Department of Defense and industry.
Images in the text are:
1. Close-up of corrosion under F/A-18F wing panels. Credit: NAVAIR.
2. Demonstrating galvanic incompatibility. Credit: Rodriguez-Santiago ASETS Defense 2018.