Corrosion can be seen all around in our daily lives. The National Association of Corrosion Engineers (NACE) in the USA defines corrosion as “a naturally occurring phenomenon commonly defined as the deterioration of a material (usually a metal) that results from a chemical or electrochemical reaction with its environment”1.
A common example of corrosion is the rusting of steel (Figure 1).
Unfortunately, thermodynamics tells us that there is a natural tendency for most metals to return to their original state i.e. the ore from which they were processed (Figure 2). For example, steel reverts to iron ore (oxide or rust), and aluminium reverts back to aluminium oxide (Bauxite). These oxides are the result of corrosion. Therefore, corrosion is inevitable for most engineering alloys in many of the environments in which they are used. The challenge in the application and use of these alloys is to prevent them from corroding.
Why is corrosion a concern in air, marine and land based structures and assets?
Firstly, corrosion damage can reduce section thickness and therefore affect the mechanical integrity of structures through the initiation of fracture and fatigue cracking. This may lead to catastrophic failure and consequent loss of life.
Secondly, it is very expensive to find, remove and repair corrosion damage, and this adds to the through life costs associated with the maintenance of assets.
Thirdly, the time spent on dealing with corrosion represents a loss of capability of a ship, aircraft or land based structure, and the consequent loss of income derived from that asset.
The Corrosion Process
These local anodes and cathodes may be microconstituents in the alloy microstructure (Figure 4), or joints crevice and fastener holes in a structure (Figure 5).
In order for corrosion to occur, these local anodes and cathodes must be in contact with an aqueous environment or electrolyte e.g. a moisture layer containing salt contaminants. At the anode metal ions go into solution releasing electrons into the metal (the corrosion event). At the cathode, electrons are consumed by a cathodic reaction at the adjacent metal surface which may be the reduction of oxygen in the environment. Hydroxyl ions produced by this oxygen reduction reaction migrate to the anode through the electrolyte to maintain an electroneutral surface and an electrochemical circuit is established (Figure 3).
Break the circuit. Prevent corrosion.
Corrosion prevention is all about breaking this circuit or preventing it from becoming established. Either through preventing access of the environment to the surface, or introducing species into the environment which act at the local anodes and cathodes and prevent the reactions from occurring there.
This basic mechanism (Figure 3) may be applied to either pitting corrosion, crevice, intergranular, galvanic, or filiform corrosion, or stress corrosion cracking. This basic mechanism helps to understand how corrosion can be controlled or prevented (i.e. breaking the circuit), and is used to explain the effectiveness of various parts of a corrosion prevention strategy for aircraft structures.
The most common way to prevent corrosion from occurring is to apply a barrier coating (paint layer) over the metal surface to stop access of the environment. A paint coating not only acts as a barrier to moisture, it also provides a high resistance pathway to the movement of ions when the barrier has been permeated by water, thus restricting the flow of ions in the electrochemical circuit as depicted in Figure 3, and slowing down the corrosion process. In the event that a coating is damaged or deteriorates over time and fails mechanically, moisture can penetrate to the exposed metal substrate. Inhibitor pigments contained in the pre-treatment layers of paint coatings are available to provide corrosion protection. These inhibitors can act to prevent corrosion by shutting down potential electrochemical reactions at anodes and cathodes on surfaces and inside crevices, pits and cracks. Inhibitors may produce protective layers on the surface either by combination with the metal to form an oxide film for example, or by adsorption of molecules onto the surface.
Corrosion will not occur until the protective coating system fails. All paints on an aircraft structure will admit moisture over time, as they degrade due to weather exposure, UV, temperature excursions from high altitude flight, soil accumulation and stressing.
Evidence of degradation is discoloration, chalking and cracking. As they age and degrade they harden, and cracking develops particularly in areas of structure where flexing can occur e.g. within the joints, inside fastener holes and on mating surfaces. The network of cracks formed will allow all constituents of the atmospheric environment to penetrate through to the substrate metal. Inhibitor ions leached from the primer and conversion coating at the point of cracking will eventually be exhausted and corrosion will occur.
In addition to painting, another corrosion prevention and control strategy for corrosion protection and arrest is the use of the common maintenance action of periodic aircraft washing using detergent products. Washing of soil deposits from a surface removes potential local anodes and cathodes where the paint coating has failed. For example, a soil deposit can create a site for crevice corrosion. Washing not only washes away salt contaminants and dirt or soil, but the surface active nature of chemicals in detergents allows them to penetrate into cracks and joints where coatings have failed. The inclusion of an inhibitor into a detergent formulation when applied to the outside of the aircraft would find its way into cracks in coatings along joints and around fastener heads, and counteract the deleterious effect of any atmospheric contaminants such as chloride ions. These ions are very effective in initiating corrosion.
Laboratory Tests with Zi-400
Early research work in the 1980s at the Aeronautical Research Laboratories (now Defence Science & Technology Group (DST)) which provides science and technology support to the Australian Defence Force, showed that various surfactant compounds commonly used in detergent formulations were very good inhibitors of corrosion on aluminium alloys. This suggested to the DST researchers that many commercially available aircraft washing detergents may have corrosion inhibiting capabilities. In 1993 at the request of the RAAF, DST investigated 19 washing detergents for their ability to inhibit corrosion1. All of these products complied with various specifications including DEF (AUST) 5570A “Cleaning Compound, Aircraft Surface (Alkaline Waterbase)”, Boeing D6-17487 REVISION P (Exterior use), Boeing D6-7127 REVISION M (Interior use), AMS 1526B (Exterior use), AMS 1550B (Interior use). The tests called up in these specifications ensure the cleaning capability of the products, and ensure that the detergents do not corrode a range of metals at a greater than a specified rate. The specifications do not assess if the cleaner can actually inhibit corrosion, or slow the growth of existing corrosion.
The DST study concluded that Zi-400 was the most effective detergent for inhibiting corrosion in very severe corrosive environments, and most importantly it was capable of arresting or retarding existing corrosion. The study also showed that Zi-400 was very effective in reducing the rate of under film corrosion emanating from defects in an epoxy primer paint coating on an aluminium alloy, and reducing crevice corrosion on an aluminium alloy.
More recently, a comprehensive programme commissioned by Solidus Industries was carried out by Deakin University’s Australian Centre for Infrastructure Durability to undertake a more in depth assessment of how effective Zi-400 HD is as a corrosion inhibitor3. For comparative purposes tests were also conducted on a well-known brand of cleaner, widely used by the aviation industry and approved for use on aircraft (Product X). The main results from this programme are summarised in the remainder of this article. These results demonstrate how very effective Zi-400 HD is as a corrosion inhibitor.
Zi-400: The Results
a. Control Solution 0.6% NaCl
b. Control Solution with 3% Zi-400 HD
c. Control Solution with 17% Zi-400 HD
d. Control Solution with 3% Product X
e. Control Solution with 17% Product X
It is clear from the Deakin University study that Zi-400 HD is a strong corrosion inhibitor for both aluminium alloys and steel, when present in a very corrosive chloride containing environment. Such environments are similar to those which can develop as thin layers of moisture on the external surfaces of aircraft and in marine structures. The study has also shown that Zi-400 HD is very effective in reducing the rate of galvanic corrosion which can occur between dissimilar metals in contact in moist corrosive environments. A most significant result of the study was that Zi-400 HD was able to dramatically reduce the rate of corrosion in an existing corrosion process to very low values. The early DST study found that Zi-400 was able to delay the initiation of filiform corrosion, and when it did initiate it propagated at a very slow rate.
As a result of the DST study, Zi-400 was shown to act as a cathodic inhibitor. That is it adsorbed strongly at cathodic sites (See Figure 3), reducing the rate of the cathodic reactions and thus reducing the rate of corrosion. This mechanism is thought to be the basis for the excellent corrosion inhibition performance of Zi-400.
The Deakin University study showed that washing detergent Product X which conformed to Boeing D6-17487 REVISION P and AMS 1526B aircraft specifications, when tested under identical conditions to those used for the Zi-400 HD tests, in some cases actually increased the rate of corrosion on both Al alloys and steel, when compared to the Control solution. It also had no effect on the corrosion rate when added to an existing corrosion process.
While regular washing of the outside of aircraft structures is mainly for the purpose of cleaning away deposited soil and airborne contaminants such as chloride, these studies have shown that washing with a detergent such as Zi-400 HD also has the effect of applying a powerful corrosion inhibitor to the outside surfaces. The surface-active properties of Zi-400 HD will allow it to penetrate into cracks in paint coatings, into fastener holes and into joints and crevices, where it can provide significant protection against corrosion. The data shown above indicate that the effectiveness of Zi-400 HD increases with increasing concentration. With regular washing and rinsing, it would be expected that concentrations of Zi-400 HD would build up within fastener holes, joints and crevices, thereby providing an ongoing level of protection. The added benefit of washing with Zi-400 HD is that if corrosion had already started in those locations, this product would act to significantly reduce the rate at which the corrosion progresses. This was strongly indicated by the galvanic corrosion tests where Zi-400 HD was added to the corrosion environment after corrosion had initiated.
All the tests performed clearly show Zi-400 HD has the unique ability to control corrosion. This reduction in corrosion would be evident in the field wherever Zi-400 HD is regularly used as part of a corrosion prevention and control programme across a wide range of industries from aviation, marine, power generation, military, oil & gas, mining and industrial.
- Zi-400 HD has been shown in the Deakin University study to be an excellent inhibitor of corrosion of aluminium alloys and steel in a very corrosive chloride containing environment.
- Its effectiveness as an inhibitor has been demonstrated by its prevention of pitting corrosion under constant and alternate immersion conditions, filiform corrosion, crevice corrosion and galvanic corrosion.
- Zi-400 has the ability to retard and arrest the growth of a corrosion process under very severe conditions. This is of particular value where the corrosion process is driven by the interaction of dissimilar metals providing a very strong driving force for corrosion.
- Zi-400 is able to provide excellent corrosion inhibition because it is thought to adsorb strongly at local cathodes on the surface thus reducing significantly the rates of cathodic reactions, and therefore retarding the overall electrochemical corrosion process.
- G. A. Jacobson, Corrosion – A natural but controllable process, https://www.nace.org/resources/general-resources/corrosion-basics
- B. R. W. Hinton, M. Cosgrave, P. Rohan and D. Luscombe, Corrosion Inhibition with Aircraft Washing Detergents, Commercial in Confidence, ARL Report No. CCR 7/93, May, 1993.
- P. Huang, J. Yan, T. Khoo and B. Hinton, Corrosion Inhibition with Aircraft Washing Detergents, Deakin University, Australian Centre for Infrastructure Durability Technical Report No. T-C0098-1, 5 August 2016.