Protection of multimaterial assemblies

1 Demands on light weight engineering and associated issues

Nowadays the complex design of modern engineering structures frequently includes various different materials in the same system. Metals and alloys are still among most used construction materials for different applications. However, the modern concepts of light design call for new solutions, especially for the transport industries. The strategies of the “light car” and“green aircraft” promoted in Europe are based on a desirable significant weight reduction of future vehicles, aiming at the lower fuel consumption and consequently also the reduction of carbon dioxide emission.

The lightweight design concept is grounded on usage of alternative materials with comparable mechanical properties and lower weight. Thus Mg alloys, Al alloys, Ti alloys, high-strength steels, and carbon fibre reinforced plastics (CFRP) are considered by automotive and especially by aeronautical manufacturers for different primary and secondary structural parts [1]. However, the combination of several different materials in the same structure raises many challenges, such as the need for new forming and joining technologies as well as surface protection processes. Among the most critical issues is the galvanic incompatibility of different conductive materials in the same assembly. Formation of galvanic coupling between the dissimilar materials can cause an unpredictable and rapid corrosion degradation of more active material. For instance, if CFRP and Al alloy are in electrical contact, the dissolution of Al alloy will occur when exposed to corrosive environment [2–4]. However, this phenomenon is inconsistent with the objective of increasing maintenance intervals and optimized life cycle.

Galvanic corrosion is an issue of great concern in both the automotive and aerospace industries. One of the ways to estimate the importance of this problem is to use simulation approaches in order to define the most critical galvanic combinations, structure designs, and exploitation conditions. The fundamentals of galvanic corrosion are generally quite well understood. However, the difference between scientific research and the performance of real system in real conditions can be notable. On the other hand, reliable simulations can also contribute to proper design of protection schemes for various scenarios appearing in multimaterials structures.

There were several attempts to model the galvanic corrosion processes in aircraft-relevant aluminum+CFRP galvanic couples. Palani et al. have tried to develop a model which can adequately describe the galvanic corrosion processes in thin electrolyte conditions that could occur in the upper part of an aircraft fuselage [5]. The numerical model was based on solving the electroneutrality equation with a three-dimensional boundary/finite element bethod. This approach can be applied to different hybrid structures relevant for aircraft frames. Specific variations in the environmental condition can be considered, including different aggressiveness of electrolytes and different thicknesses of electrolyte film. The use of multimaterial mixes is becoming increasingly important in automotive body design. There are several relevant combinations which can lead to serious galvanic corrosion issues. The Al+CFRP galvanic couple discussed above and all the respective corrosion issues are also relevant for automotive structures.

One of the attractive materials to be integrated within the car body is magnesium becauseof its low density andconsequently its high weight-saving potential. However, the galvanically induced corrosion of magnesium alloys is a primary concern in this case. Of all the structural metals, magnesium has the most active galvanic potential and suffers serious galvanic corrosion when coupled with a different metal or CFRP. Jia et al. have used a similar boundary element method (BEM) simulation approach to model the galvanic corrosion of magnesium alloy AZ91D coupled to a steel fastener [6]. The total corrosion rate of the Mg part was composed by two additive components consisting of galvanic corrosion and self-corrosion. The experimental measurements were performed in order to validate the model and demonstrate good agreement between the experiment and BEM model. The model has predicted a similar distribution of the current density for AZ91D+steel galvanic couple: a maximum at the interface with quickly decrease to zero within 1 to 2 cm from the interface, as shown in Fig. 1.

Fig. 1

Comparison of the galvanic corrosion area for BEM model and the immersion test for 10 mm diameter steel cathode: (a) experimental immersion test; (b) the BEM model [6].

The obtained results clearly demonstrate that utilization of steel fasteners can be critical when joining the Mg alloys. AA6xxx fasteners, for example, cause significantly lower galvanic corrosion of Mg during salt spray tests [7]. An important factor in this case is a significantly lower difference of galvanic potentials between the fastener and Mg structure. Galvanic coating of steel with more active metallic layers can be also used in order to reduce the galvanic couple in the case when steel fasteners or bolts are not avoidable [8]. Zn-based Sn+Zn metallic coatings [9] have reasonable galvanic compatibility with Mg alloys when exposed to salt-containing electrolytes. The application of additional polymer coatings to the fastener (or bolt) surface can also be considered as a potential approach [10].

Different joining technologies can be applied for hybrid structures including above mentioned mechanical fastening, adhesive bonding, laser-beam welding, and friction stir joining [11].

One joining approach is the laser weld brazing (LWB) which can be used to galvanize steel with aluminum components by the use of either zinc- or aluminum-based filler alloys [12]. This laser-based joining technology has a shorter processing time, good adaptability, and a good weight reduction/cost ratio in serial production when compared to adhesives and rivets for steel-aluminum mixed structures. However, the corrosion of used filler material can be significantly accelerated by galvanic coupling with respective substrates causing accelerated degradation of the brazed seam. Moreover, long-term galvanic effects are possible in a good conductive medium.

The testing of multimaterial assemblies in order to understand their susceptibility to galvanically induced corrosion attack is another important challenge. The standard electrochemical methods or accelerated corrosion tests are not always applicable. For example, the interpretation of electrochemical impedance spectroscopy results obtained on galvanically coupled multimaterial samples is not straight forward, since the contribution from both substrates are difficult to separate. Moreover, the linearity requirement is not always achieved because of the remarkable galvanic currents passing between the anode and the cathode. The test samples for accelerated corrosion tests should be also designed in a different way when compared to the single-material samples. One of the test sample geometry for brazed steel+Al sample is demonstrated in Fig. 2.

Fig. 2

Arrangement of brazed multi-material specimen for salt spray test, adopted from [12].

It is important to notice that the type of joining method and the sealing of the joint can drastically influence the results in accelerated corrosion tests, since only relatively thin films are present on the surface during the test. Thus the geometry of the joint or its positioning in the chamber will influence the liquid film formed and its conductivity, which is decisive for galvanic coupling. The geometrical factors such as distance between the cathode and anode, cathode/anode area ratio, the thickness of the electrolyte film, and the shapes of the electrodes are important [13]. Moreover, several galvanic couples can be present in the same structure, e.g. several steel fasters for one magnesium part. There can be an interaction/superimposition of the currents resulting from each galvanic couple. One of the ways to test the galvanically coupled materials and understand the geometrical effects on the local galvanic currents is to use multielectrode assemblies with numerous electrodes, which can be coupled and decoupled on demand. A multielectrode arrangement to study the Mg+Fe galvanic couple is demonstrated in Fig. 3.

Fig. 3

(a) Schematic of idealised one-dimension galvanic couple. (b) Section through multielectrode Mg–steel galvanic corrosion assembly (GCA), adopted from [13].

The multimaterial approach also multiplies the amount of the required testing time, especially when new corrosion inhibitor systems are to be developed. Therefore the systematic high-throughput concept has been considered [14] by Taylor et al. The multielectrode arrays of identical AA2024-T3 pairs were immersed in numerous separated reaction cells, and a potentiodynamic analysis qA performed for estimation of polarization resistance. A large amount of different corrosion inhibitor solutions were tested simultaneously. More recently Mol et al. [15] has proposed a multielectrode assembly which contains several different materials in the same epoxy mount and could be immersed in the same testing solution for simultaneous potentiodynamic analysis (Fig. 4).

Fig. 4

Face view of multielectrode assembly showing the layout of wire specimens (a) and schematic of experimental setup for an individual electrode pair [15].

Although the DC-polarization technique is destructive for the electrode surfaces, and additionally due to the electrochemical excitation, cross-contamination of different materials in the test cell may also occur. Therefore a noninvasive localized measurement technique SVET (scanning vibrating electrode technique) has been considered as a high-throughput electrochemical method for simultaneous corrosion inhibitor screening for multimaterial applications. It measures the localized corrosion-related ionic currents and allows observation of the distribution and evolution concurrently for both anodic and cathodic processes. Also the conditions during the SVET operation are close to an undisturbed environment, as the natural corrosion process is not influenced by any applied polarization. Historically the SVET method was initially applied in the life sciences [16–19], and only later introduced in corrosion research [20–22]. SVET has also been coupled with a wire beam electrode (WBE) by Battocchi et al. (Fig. 5) [23]. The designed model cell contains eight Al and one Cu electrodes, which are electrically connected and could therefore emulate the galvanic situation on AA2024-T3 surface (Fig. 5). In Fig. 6 the ionic current distribution on this electrode is presented. It can be seen how the localized cathode on Cu and anodes at Al electrode were formed.

Fig. 5

SVET measurement set-up, adopted from [23].

Fig. 6

Current density map (left) and optical micrograph with current density vectors (right) of 8Al–1Cu WBE after 1h of immersion in 0.35 wt.% (NH₄)₂SO₄ + 0.05 wt.% NaCl solution, adopted from [23].

Reproducing the film electrolyte conditions over galvanically coupled materials is an even more challenging task, especially when a controllable film thickness is required. Recently Hack et al. patented a method and device which allow precise control of the electrolyte film thickness [24]. The arrangement of the respective test sample and the testing setup are shown in Fig. 7. The formation of a uniform thin film is based on the theory of Nusselt for falling film-wise condensation on a vertical flat plate. In general, the condensation of the electrolyte mist begins at the top and moves downward with the force of gravity, creating a laminar flow. The tested substrates are connected as electrodes to the ZRA, which allows in situ measurement of the galvanic current passing between the cathode and the anode [11]. In this way, different material combinations can be tested under condensing conditions, which are relevant for many applications of hybrid structures.

Fig. 7

Schema of the setup used for thin film tests of multimaterials assemblies, adopted from [11].

The understanding of the galvanic corrosion issues for a specific combination of materials, relevant geometries, and conditions is an important starting point for the proper design of multimaterial structures. Moreover, the knowledge of these issues creates a basis for the further development of protection strategies. The main ways used for protection of multimaterial structures are briefly overviewed in the next section, with particular focus on novel active protection approaches.

2 Approaches to mitigate corrosion issues in multimaterial systems

2.1 Passive protection

The galvanic corrosion occurs only in the case where three important conditions are met: (1) a significant difference of the galvanic potentials of joined materials, (2) electrical contact between the dissimilar materials, and (3) the presence of electrolytes in simultaneous contact with both materials. The main passive protection methods for hybrid structures are based on strategies which try to eliminate one of these factors from the equation.

The first logical approach to prevent the galvanic corrosion issues is to avoid the electrical contact between the dissimilar materials in a hybrid structure. Most of the joining methods create a direct contact between the materials, allowing electrons to pass easily. However, the use of adhesive bonding can eliminate the electrical contacts. In this situation the acceleration of corrosion processes due to the galvanic coupling can be avoided. In most of the structural applications the pure adhesive bonding is rarely applied, especially in the aeronautics industry. Instead, hybrid joints are preferred which combine the adhesive bonding with additional mechanical fastening. In this case electrical contact is created, and the galvanic corrosion issue arises again. In the case of aeronautical applications, one more obstacle in adhesive bonding may occur as the electrical contact between all structural parts of aircrafts is desired in order to avoid the problems of static discharging and lightning strikes.

Another possibility is to reduce the difference of the galvanic potentials between the materials used in design. This possibility is often impossible, since selection of the materials is defined by the required mechanical properties, weight, thermal conductivity, etc. Therefore, flexibility in the choice of materials is often limited. In some situations the potential difference can achieve extreme values, as in the case of Fe+Mg or CFRP+Al multimaterial mixes. The difference in the galvanic potentials is the main driving force for accelerated corrosion. However, in the case of low electrolyte conductivity or very thin electrolyte layer, if dissimilar materials are positioned in the structure together, the potential drop can be significant. Thus the galvanic current can be drastically reduced. Use of insulating spacers in critical zones is not always possible. Therefore another metal with intermediate galvanic potential is often introduced as a spacer between the noble and active components. In the case of aeronautical structures Ti is often used for a transition between CFRP and Al alloys. In the case of automotive structures, Al alloy spacers can be applied between of Mg alloy and steel components, in order to avoid a sharp change of galvanic potential. Fig. 8 demonstrates the design of the hinge connection to the Mg part of a car door. The aluminum alloy is introduced between the steel and Mg parts in order to reduce the local potential difference.

Fig. 8

Attachment of the steel hinge to the Mg inner part via an aluminum plate [25].

Application of sealants and barrier coatings is one of the main strategies utilized in protection of dissimilar joints against galvanic corrosion. In both automotive and aeronautical structures the critical zones are often sealed with polymer sealants, which prevent the direct contact of moisture with both dissimilar parts. The strategy functions well in areas where only a very thin electrolyte film can be formed on the surface. The sealing is also very important to protect the occluded zones susceptible to the accumulation of electrolytes. However, the sealing of the joint area is not sufficient in the situation where hybrid structures are used in immersed conditions, such as the bottom internal part of an aircraft fuselage, where a significant amount of liquid can be accumulated during operation. The application of the protective coatings can be considered to be an ultimate solution. The coating creates a physical barrier between the electrolyte and substrate. Thus no corrosion can occur as long as there is no failure of this ideal barrier. The problem is that different coating systems and various often incompatible technologies are applied for different substrates, with the first complications already arising at the cleaning/pretreatment stage. Conditions suitable, for example, for aluminum alloys can be too harsh for magnesium-based materials. Therefore, pretreatments or even some coating layers can be applied before the joining and the final coating is done over the joined and partially precoated components. An example of such process suggested for the Mg+Al rear door of a Volkswagen car is demonstrated in Fig. 9.

Fig. 9

Schematic representation of the coating process for Al+Mg hybrid structure, adopted from [26].

The magnesium panels are first degreased and rinsed. At the second step about 20–30 μm of the casting crust is removed by acidic etching. Then the surface is rinsed, chromate, and dried. All the chemicals are applied using the spray process. The eCoat is applied in the next step and cured for 15 min at 180 °C. Finally, a powder coating is applied and baked at 200 °C for 10 to 15 min.

The surface of aluminum panels is standardized by acidic pickling. Hybrid bodies are then formed and bonded. In the next processing step, the hybrid body is put through the eCoat in the steps of degreasing, phosphatization, eCoat application, and baking at 180 °C for 14 min. Only the aluminum sheet in the process absorbs the eCoat, while the magnesium panel is electrically insulated by the coating powder [26]. As one can see the total process is significantly more complicated than the one used for a conventional car body. This fact leads to unfavorable cost aspects, especially for mass production cars. The ideal scenario would be the application of the same surface treatment process for the entire multimaterial car body composed of different steels, Al alloys, Mg alloys, and reinforced composites. Recent developments have focused on surface treatment technologies which can be applied for multimaterial substrates. For example, tuning the electrolyte composition and the electrical parameters of plasma electrolytic oxidation allows the formation of anodic coatings on Al (AA7075) and Mg (ZE41) substrates. Microdischarges were sustained on both metals for the optimized conditions, and coatings were produced with thicknesses reasonably similar to those of the coating on metals treated individually [27].

The important issue is that even when an “ideal” coating is applied to the hybrid structure, with time the barrier can be disrupted during use and certain defects may appear. A rapid degradation reinforced by galvanic coupling can then occur in the coating defect. Fig. 10 demonstrates a coated Mg alloy sample galvanically coupled to uncoated steel. One can clearly observe an intensive paint delamination and corrosion of Mg substrate underneath. Thus, even a well-coated multimaterial assembly can quickly begin to degrade when defects appear in the protective coatings. Moreover, the corrosion process can be of a very localized nature and lead to unpredictable failure of the respective part.

Fig. 10

Coated Mg alloy coupled to steel after 30 cycles according to the Volkswagen testing regulation PV 1210 [26].

2.2 Active protective coatings with corrosion inhibitors

The introduction of active inhibiting compounds to the protection system is a promising approach which can add active protection to the passive protection methods reviewed above. Corrosion inhibitors can be introduced in hybrid structures incorporated in coatings, sealants, or structural adhesives. In the two last cases the inhibitor is placed as near as possible to critical places such as confined environments and multimaterial joints. In the case of aeronautical applications a chromate conversion layer and chromated primer provide very effective corrosion inhibition ability for Al alloy structures [28, 29]. However, present regulations impose restrictions on the use of chromates, and therefore new solutions are needed. Thus, the search for effective anticorrosion coatings containing self-healing active species which could replace chromates has been very much in the forefront [30–33].

The corrosion inhibitor can be introduced directly to the coating, but in that case it must be compatible with the coating matrix, and it should be able to reach the active surface in order to suppress corrosion. In the case of sol-gel and silane-based coatings several successful systems have been discussed [33–35]. There are even some specific cases recognized where the coating barrier properties have been improved with the introduction of inhibitors. However, there is always a risk of the uncontrollable leakage of the inhibitor from the coating and loss of its long term effectiveness [36]. Moreover, usually the stability and barrier properties of coatings will be significantly decreased due to the unwanted interaction of corrosion inhibitors and coating formulations [37]. Application of highly soluble inhibitors can also give rise to issues around strong osmotic blistering [38].

In order to diminish these negative effects, entrapping the corrosion inhibitor in nanocontainers has been suggested. This approach can protect the coating formation during the paint application process from the destructive influence of corrosion inhibitors. Later these nanocarriers, if they have specially designed functional properties, can release corrosion inhibitors on demand. The container release function can be triggered in various ways to react to different corrosion relevant stimulus (pH, chlorides, stress, mechanical damage, or UV light) [37, 39–41]. Recently several works appeared suggesting various possibilities for selecting and designing containers for corrosion inhibitors, for example oxide nanoparticles [34], β-cyclodextrin fillers, plasmapolymer shells [42], cellular nanocontainers (diatomaceous earth, zeolite, or carbon-based) [43], and ion (anion [44] and cation [45]) exchangers. Another promising possibility is to prepare nanoreservoirs with “smart” storage/release properties, using the polyelectrolyte layer by layer (LbL) assembled shells, which change the permeability depending on the pH and ionic strength [37]. The layered double hydroxides and mesoporous nanoparticles have also proven to be promising candidates for the encapsulation of different corrosion inhibitors [46, 47].

The situation is even more complicated and demanding in the case of multimaterial substrates, where galvanic activities beneath the coating could be suspected. One possible option to deal with this is the incorporation of different functional nanocontainers for each of the substrate materials, which is also more thoroughly discussed in the present chapter, for instance one nanocontainer system dedicated to the suppression of a cathodic reaction and another one for an anodic site. The synergistic effect of encapsuled corrosion inhibitors is also desired [48]. Different inhibitors used together can confer efficient inhibition at different conditions, one working at anodic (acidic) conditions, and another at a cathode, where high pH levels are expected. This approach is especially relevant in the case of coatings for multimaterial structures where a clear separation between cathodic and anodic sites happens due to galvanic effects. Using several inhibitors in the same system opens opportunities for cooperative actions between them resulting in synergistic inhibition effect. This can allow significant reduction of the minimal critical inhibitor concentration needed to ensure suppression of corrosion processes in defects. Only a small number of papers have been published recently which focus on this aspect. The best synergistic combinations of corrosion inhibitors are yet to be found. There are two main possibilities of how to implement this strategy in coatings: to use a low-soluble complex pigment composed of two inhibiting constituents (cerium dibutylphosphate (Ce(dbp)₃) [49], praseodymium diphenylphosphate (Pr(dpp)₃) [50], rare earth + mercaptoacetate [51], cerium cinnamate [52], cerium molybdate [53, 54]); to introduce two inhibitors in the same coating system separately in an encapsulated form (double doped zeolites [55], LDH-BTA + Bentonite-Ce [48]).

The next part of the chapter concerns special example of the successful development of an active protective coating for galvanically coupled multimaterial systems. Different important steps are shown in order to suggest a road map which can be followed for specific hybrid structures.

Step 1: Exploring synergistic mixtures of corrosion inhibitors

Galvanic corrosion is generally a very challenging phenomenon for corrosion inhibitors as the thermodynamic driving force to corrode can be very high [56]. Therefore a majority of the current inhibitor systems are mostly only able to efficiently protect single material structures. In the case of aeronautical grade Al alloys several intermetallic phases such as the widely investigated Al-Cu-Mg second phase behave like a galvanic systems at the microlevel [57, 58] and for this particular case several highly efficient solutions were developed. The usage of chromates [28, 29] for aluminum alloys has been an efficient and robust solution, but also the selection of other, more environmentally friendly and less toxic potential candidates is large [59]. However, when the Al alloy is coupled with more noble conductive material like CFRP, advanced synergistic corrosion inhibitor mixtures are desired.

Recently the synergistic behavior of two known corrosion inhibitors, 1,2,3-benzotriazole (BTA) and Cerium (III) nitrate (Ce(NO₃)₃), has been demonstrated to be a suitable active protective combination for an Fe + Zn galvanic couple in a chloride medium [60].

BTA is among the efficient corrosion inhibitors for different metals, especially for copper and its alloys, and has have been well known for more than sixty years [61]. Mostly it acts as a mixed-type inhibitor through physical and chemical adsorption as well as different Cu-BTA complex formation mechanisms providing its predominant effect on inhibition of anodic corrosion reaction [62]. One possible structure of adsorption BTA adsorption layer on Cu surface is described in Fig. 11.

Fig. 11

Schematic presentation of chemisorbed layer formed by 1,2,3-benzotriazole adopted from [63].

It is also known that BTA can form protective coatings with Zn [64–66] and Fe [67]. In some circumstances it has been found that BTA can cooperate synergistically with other chemical species like benzylamine [68] and sodium dodecylsulphate (SDS) [69]. The synergism with iodide anions was also recently reported [70]. It is based on the coadsorption of I⁻ and BTA, which somewhat improves the complex formation with Cu or Fe [67]. However, all these findings about BTA and synergistic cooperation was discussed for suppression of single material corrosion, while the above mentioned combination with Ce(NO₃)₃ has been especially designed for galvanic corrosion. The cerium cations (Ce³⁺) are known as cathodic inhibitors. Similarly to some other rare earth metal (La³⁺, Y³⁺) cations they form blocking hydroxide precipitates (reaction 3) due to local pH increase at cathodic sites (reactions 1 and 2) [71–75].

(1)O2 + 2H2O + 4e− → 4OH−

(2)2H2O + 2e− → 2OH− + H2

(3)Ce3+ (aq) + OH− (aq) → Ce(OH)3(s).

The further oxidation of Ce(OH)3(s) leads to the formation of CeO2 according to the reaction (2). This compound is also insoluble and prevents the contact of the surface with the aggressive environment [75]:

(4)4Ce(OH)3 + O2 → 4CeO2 + 6H2O.

However, the inhibition efficiency in this case also depends significantly on the intensity of cathodic corrosion reaction, as the forming deposits may possess different structures and composition, which can change the corrosion blocking barrier properties.

The driving idea to combine BTA with cerium nitrate for Zn+Fe galvanic couple has been to find one suitable inhibitor for anodic and another for cathodic reactions.

Experimental approach for evaluation of the synergistic effect

The localized technique SVET coupled with a multielectrode cell has been also applied for corrosion inhibitor screening simultaneously for different single materials [76], but the great advantage of this approach is the possibility to adequately and systematically observe the galvanic corrosion activities when the model electrode with the selected galvanic couples is immersed in different corrosion inhibitor testing solutions or mixtures (Fig. 12).

Fig. 12

SVET setup for inhibitor testing on galvanic model system.

Using the above described experimental set-up, a screening of corrosion inhibitors was performed for the aeronautically relevant CFRP+Al galvanic couple. The results for two different corrosion inhibitors, which were discussed also above in the context of Zn+Fe galvanic system are: 1,2,3-benzoriazole (BTA) and cerium (III) nitrate were presented in Fig. 13. The micrograph of the test-cell and SVET measured corrosion current density maps were all taken after 3 h of immersion in the selected corrosion inhibitor environment. The results clearly demonstrate that the addition of 5mM Ce(NO₃)₃ inhibitor in 0.05 M NaCl electrolyte slightly decreases the cathodic and anodic corrosion activities in comparison to the reference NaCl solution (Fig. 13(d)). With the 1,2,3-benzotriazole the process was even somewhat accelerated. However, the combination of these two inhibitors leads to significantly more efficient corrosion suppression on Al+CFRP couple. Only some hardly detectable electrochemical ionic currents can be seen in Fig. 13(e).

Fig. 13

Microphotograph of Al-CFRP galvanic test cell (a), SVET maps taken after 3 h of immersion in different inhibitor solutions with Al and CFRP electrodes electrically coupled in 0.05M NaCl (b), and with addition of inhibitor 5 mM BTA (c), 5 mM Ce(NO₃)₃ (d), and the combination of inhibitors 2.5 mM BTA + 2.5 mM Ce(NO₃)₃ (e) [48].

With more detailed analysis some additional information about the inhibitor efficiency and functioning kinetics can be also achieved. In Fig. 14 the same series of experiments as in Fig. 13 is described. The evolution of maximal anodic and cathodic ionic current densities (detected by SVET) is shown. It can be seen that both the anodic and cathodic currents in BTA+Ce³⁺ containing 0.05M NaCl testing medium decrease rapidly during the first hour of immersion and also stay very close to zero during further testing. However, the same inhibitors alone are unable to stop the galvanic corrosion on an Al+CFRP couple.

Fig. 14

Evolution of anodic and cathodic maximal ionic currents for different corrosion inhibitor systems on Al+CFRP galvanic couple.

Calculation of corrosion inhibition parameters from SVET values

Based on the experimental results several characteristic parameters can be calculated. SVET as a localized technique has been historically mostly applied for qualitative analysis of ionic currents, but with careful experimental set-up and experimental design the qualitative information will be also available. Moreover, especially for studies of galvanic corrosion model systems there are many advantages over the classical electrochemical corrosion rate estimation methods [48, 60, 76, 77].

Maximal anodic and cathodic currents

The SVET measured localised ionic current density values can be an adequate reading of corrosion at a selected measurement location point. Since the results can be presented as three-dimensional spatial localized current maps, the maximal and minimal values over the map usually can point very well to the most active zones. Therefore the maximal anodic (I_{max AN}) and maximal cathodic (I_{max CAT}) current density values are the first and simplest indicators for the characterisation of corrosion activity at a selected sample.

In the case of a galvanic couple, when the single material corrosion separately from galvanic system does not take place, usually tese values are strictly allocated to the cathode and anode in the reaction cell. Therefore a good characteristic for the whole galvanic couple could be the summarized maximal ionic current density (I_sum), which is the sum of the absolute values of cathodic and anodic maxima:

(5)Isum = |Imax  CAT| + |Imax  CAT|.

Integrated ionic currents

An advanced approach is the integration of SVET measured ionic currents. As with the scanning of a SVET probe, the ionic current measurement is not continuous, and the map is presented as a raster of current density values the integration of these values presumes the initial differentiation to the real allocated surface area related to each data-point. For a characterization of a galvanic couple the integration of the absolute values will again be the best solution, as it counts both the anodic and cathodic currents for the same system:

(6)Iint = ∑n=1N|in| ⋅ Sn,

where |i_n| is the absolute value of the SVET current density measured at point n (at a certain distance from the surface (usually 100–200 μm)), S_n is the surface area (cm²) corresponding to a single measurement point, and N is the total number of data points included in the calculation.

Inhibition efficiency (IE)

The parameter for estimating the inhibition efficiency can be found as

(7)IE = CR0 − CRinhCR0,

where CR₀ is the corrosion rate in the noninhibited medium and CR_inh is the corrosion rate in the presence of inhibitor. In the case of the SVET data the I_sum and I_int are proportional to the corrosion rate.

Synergistic parameter (S)

The parameter that numerically expresses the synergistic effect of two corrosion inhibitors can be found as suggested by Aramaki and Hackerman [78]:

(8)S = 1 − IE1+21 − IE12,

where:

(9)IE1+2 = (IE1 + IE2) − (IE1 ⋅ IE2).

The parameters IE₁, IE₂ and IE₁₂ are the calculated inhibition efficiencies for inhibitors 1, 2, and the mixture of 1 and 2, respectively. The values S > 1 indicate the synergistic behavior of a selected inhibitor combination, while S = 1 refers simply to the additive effect of inhibitors, and S < 1 is the case with antagonistic behavior of selected inhibitors.

Calculation results for selected corrosion inhibitor mixtures

For an Al+CFRP galvanic systemthe inhibition efficiencies (IE) and synergistic parameter (S) were calculated on the basis of the sums of maximal (I_sum) and integrated (I^int) ionic currents, as presented in Table 1. The difference between the two calculation approaches can be noticed, while the parameters S (8.48) and S^int (2.77) are slightly varied. However, as the values are clearly higher than unity (S >1) in both cases, the evidence of the synergistic cooperation of Ce³⁺ and BTA on an Al+CFRP couple in NaCl corrosive media is proved.

Table 1

Calculated inhibition efficiencies (IE) and synergistic parameter (S) for Al+CFRP galvanic couple using maximal (I_sum, IE, S) and integrated (I^int, IE^int, S^int) ionic current values [48]

Al+CFRP
Solution	Isum/μA cm−2	IE	S	Iint/mA cm−2	IEint	Sint
Base solution NaCl	93.8	–		15.1	–
+ BTA	127.9	− 0.36	8.48	12.1	0.197	2.77
+ Ce(NO3)3	46.8	0.50		5.78	0.617
+ Ce(NO3)3 + BTA	7.5	0.92		1.67	0.889

Calculations based on maximal SVET detected signal can be overestimated due to the unevenly distributed corrosion activities on the surface, which can result in multiple current maxima, as was noticed also in Fig. 13(c). It shows that the comparison of parameters from these two approaches could also give some additional information about the grade of corrosion localization (possible pitting activities, for example), because in that case the multiple peak maxima can result in proportionally lower input parameters for calculation, while the integrated current values are obviously closer to the real situation.

An important factor to be taken into account when finding the synergistic inhibiting mixtures is the concentration ratio between the components. The identification of inhibitor synergy and the characterization of many variables that affect inhibitor performance (e.g., concentration, pH, T, etc.) cannot be predicted and must be determined experimentally. The influence of concentration on the synergistic effect of two inhibitors can even lead to the situation when an antagonism is observed, as demonstrated below (Fig. 15).

Fig. 15

Schematics representation of metal corrosion rate versus composition of a binary inhibitor system (A:B) at a fixed pH (adapted from [14]).

Step 2. Encapsulation of corrosion inhibitors

After the efficient corrosion inhibitors orsynergisticmixtures of inhibiting compounds are found, the next important stepis the introduction of the inhibitors into the real protective system. The direct addition of inhibitors in polymer formulations often leads to adverse effects, as mentioned above [79, 80]. Moreover, the release of inhibitors in such cases is hard to control, which can be critical for synergistic mixtures since an unfavorable concentration ratio between two components can be achieved. The synergism can become an antagonism in such a situation. This is the stage when the application of nanomaterials can be a way to offer a solution. The encapsulation of corrosion inhibitors in the form of nanocontainers can reduce the adverse effects on polymer matrix and ensure a controllable release on demand. In this particular example two types of functional nanocontainers were selected: layered double hydroxide (LDH) and bentonite as functional cationic and anionic ion exchangers, respectively. These nanoclay-based containers could confer a controllable delivery and functional release of inhibitors by appearance of corrosion related species like Cl⁻ or OH⁻ in the case of LDH [81, 82] and Meⁿ⁺ for bentonite [45].

Layered double hydroxides

Layered double hydroxides are known as hydrotalcite-like anion exchangers [81, 82]. This compound is formed with a layer-by-layer structure. Between two positively charged layers of metallic cations/hydroxides there are negatively charged layers of anions. Using the LDH capability for anionic exchange of some corrosion inhibitors, like 2-mercaptobenzothialzole (MBT) or BTA, it could be incorporated into the structure in anionic form (Fig. 16).

Fig. 16

The intercalation of the anticorrosion inhibitor (BTA) into the structure of LDH.

When LDH is loaded with inhibitors and inserted into a protective coating structure, it is supposed to be placed near a metallic surface. When corrosion of the substrate occurs the formation of hydroxides begins concurrently. Then the anion-exchange capability of LDH is used again, as previously intercalated inhibitors can be replaced with cathodically formed corrosion products (OH⁻), or even by invading corrosion agent e.g. Cl⁻ (Fig. 17).

Fig. 17

Anion exchange reaction occurred with LDHs during corrosion protection mechanism in active self-healing coating [83].

This leads to the main advantages of LDH use: (1) the controlled release of the inhibitor, and (2) the absorption of chlorides, whose presence is critical for the corrosion processes.

Bentonite

Bentonite is a natural clay mineral that can be used as a cation exchanger. It consists of negatively charged alumosilicate sheets, between which the inhibiting cations can be intercalated [45] (Fig. 18).

Fig. 18

Bentonite structure (adopted from [84]).

The release of the initially incorporated Ce³⁺ inhibitor can be triggered by metal cations which are available in the locations of anodic corrosion process.

Step 3. Application of multifunctional coating

The next important stage is the introduction of encapsulated inhibitors into the polymer formulation. The dispersion of nanocontainers can be an issue. Bad dispensability can cause formation of agglomerates, which in turn negatively affects the barrier properties of the coating. In this particular example bicomponent epoxy resin [48] was used as a coating formulation. Fortunately the dispersion of the nanocontainers was reasonably good, and no further surface modification was needed. The obtained nanocontainer-containing polymer formulation can then be applied using different appropriate techniques in order to get a uniform nanocomposite coating. The formulation with 2 wt.% of nanocontainers was created. The coating to be tested was applied by a spiral bar coater with 40 μm wet coating thickness on top of a model galvanically connected Al+CFRP planar substrate [48].

Step 4. Verification of active protection

The active protection functionality has to be verified after the coating is developed for a specific multimaterial combination. One of the possibilities is to create artificial defects in the coating and observe the kinetics of corrosion processes in these local zones. The systematic localized corrosion monitoring approach with SVET has been applied to monitor corrosion protection properties of nanocontainers impregnated coating. The self-healing ability was tested at artificially induced needle defects, each located on both sides of an AA6061+CFRP galvanic couple (Fig. 19(a)). The area beyond the scan was isolated by wax to exclude any possibility of uncounted electrochemical activities, and the ionic current densities were monitored for up to 24 h of immersion in 0.05 M NaCl corrosive medium (Fig. 19).

Fig. 19

Schema of AA6061+CFRP microelectrode setup (a), microphotographs of coated galvanic model-cell (b). The SVET maps for the sample with blank epoxy coating (c) and for the coating loaded with combination of nanocontainers (LDH-BTA + bentonite-Ce³⁺) (d) obtained after 2 h, 12 h and 24 h of immersion in 0.05 M NaCl [48].

The typical SVET maps obtained in the case of two coatings, the blank one (Fig. 19(c)) and the one containing a combination of nanocontainers (Fig. 19(d)) are presented. It can be seen that the anodic and cathodic activities are well defined and located strictly at the artificial defect zones over AA6061 and CFRP, in both cases at the beginning of immersion. The blank system demonstrates relatively stable values of corrosion currents in the defects, while in contrast a well-defined self-healing effect is observed in the case of the nanocontainer impregnated coating. The activity at the defect zones significantly decreased with time.

The average stabilized values of the anodic and cathodic ionic current densities at the defect zones observed between 15 h and 24 h of immersion are presented in Table 4.2. The system doped with LDH-BTA nanocontainers shows somewhat lower activity than the blank coating, while the coating with Ce³⁺-bentonite demonstrates even a slight increase of corrosion activity in the defect zones. The increase of activity can be related to a negative impact of the Ce-containing pigment on the barrier properties of the coating, which is a commonly observed phenomenon [34].

Additionally the coated AA6061+CFRP galvanic cells were inspected during their immersion in 0.5M NaCl in respect to the appearance of visible evidence of corrosion products or defects. In Table 2 the elapsed times without visible signs of corrosion (no corrosion) are also reported. It can be seen that the blank reference coating rapidly degraded during the first 12 h, while the coating doped with the combination of nanocontainers demonstrated the highest corrosion resistance: 408 hours until visible deterioration. This also clearly exceeds the coatings doped with only BTA⁻ or Ce³⁺-loaded nanocontainers, which showed corrosion signs already after 192 h and 48 h, respectively.

Table 2

Time of immersion before appearance of visible corrosion defects (No corrosion) and SVET parameters (anodic (I_AN) and cathodic (I_CAT)) for AA6061+CFRP model substrates with coating [48].

AA6061+CFRP with coating
Sample	No corrosion / h	IAN/μA cm−2	ICAT/μA cm−2
Blank coating	12	12±1	− 8.9±0.5
+ Mg(2)Al-BTA	192	5.4±0.4	−8.5±0.7
+ Bentonite-Ce3+	48	14.7±1.3	−9.8±0.7
+ Mg(2)Al-BTA+Bentonite-Ce3+	408	2.1±0.2	−1.1±0.1

The observed self-healing effect can be clearly correlated to the synergistic inhibition effect of the mixture of cerium cations and benzotriazole released from bentonite and LDH, respectively.

3 Concluding remarks

The brief overview presented in this chapter demonstrates that galvanic compatibility is an issue which can significantly shorten the life of hybrid multimaterial structures and can lead to unexpected premature failures. The design of structures, the correct choice of joining methods, and the selection of proper materials can reduce the risk of galvanic corrosion. Currently many technological solutions in the directions discussed have been developed to cope with this issue. However, most of these solutions assume the additional application of different sealing or coating strategies in order to limit the interaction of joined materials with moisture and corrosion agents coming from the environment.

The coating technologies applied for single materials are being intensively transferred to the hybrid structures, althoughwith many complications and important limitations. The compatibility of the surface treatment process developed for one material can be absolutely incompatible with another material in a hybrid structure. The new path is to develop new surface treatment processes which can be applicable to multi-material substrates. This can drastically improve the economical perspectives for multimaterial designs, especially in the automotive industry. However, passive protection relying only on the barrier properties of sealants and coatings cannot be considered as a final reliable solution. Different types of defects can appear during exploitation leading to the degradation of barrier properties and unpredictable corrosion of joined structure in such zones.

The introduction of an active protection concept via the incorporation of “smart” nanocontainers into protective coatings or sealants seems to be a promising approach which allows control over the galvanic corrosion processes in multimaterial structures. The novel nanocomposite coatings can effectively respond to any change of environment or to the beginning of corrosion processes by the release of corrosion inhibitors on demand, supressing the corrosion activities. The same approaches based on active inhibitor-loaded nanocontainers can also be used in sealants and adhesives.

The development of such “smart” protective nanocoatings for multimaterial assemblies is a multistep process which should follow a certain procedure in order to achieve a desirable performance. At the first stage the mechanisms and the conditions associated with the galvanic corrosion processes in a specific assembly should be understood. Later the efficient corrosion inhibitors should be selected for each particular case. The minimal and maximal concentration of the inhibitors as well as potential synergistic combinations should be explored. The use of synergistic mixtures can ensure more efficient active protection even with lower inhibitor concentrations. The inhibitors in most cases have to be encapsulated. Usually the functional nanocontainers are preferable to microreservoirs, especially in the case of thin coatings where the distribution of such pigments can be critical. The integration of nanocontainers into the protective system is another challenge, and often some additional surface modifications of nanoadditives have to be performed in order to ensure good compatibility. After completing all of these steps, it is possible to have a powerful tool for the design of an active protective strategy for a multimaterial structure. The advantage of such an approach is that active elements can be combined in different ways in order to ensure a reliable protection for a wide range of material combinations. Ideally, one multicomponent active protection scheme can be applied for multimaterial structure such as a complex car body.