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2019 Department of Defense – Allied Nations Technical Corrosion Conference

TOWARDS UNDERSTANDING OF CORROSION ON COMPOSITE AIRCRAFT

J.J.H.M. van Es, Royal Netherlands Air Force

L. ’t Hoen-Velterop, Royal Netherlands Aerospace Centre NLR

Keywords: Galvanic corrosion, helicopters, surface ratio, composites, static electricity, sea salt aerosols

ABSTRACT

The modern generation helicopters and airplanes have a major part of their fuselage constructed in carbon fiber reinforced plastics. Even though these new structures themselves are not sensitive to corrosion and require far less maintenance than traditional aluminum structures, corrosion can still be a problem for the aircraft. Owing to the very large cathodic surface of the structure the (anodic) metallic aircraft systems are at a high risk of corrosion if not properly protected. This is especially true when the aircraft is operated in maritime conditions.

It is well known that the surface area ratio has a pronounced influence on the galvanic corrosion rate. The extent of this effect in the situation of a carbon composite aircraft structure in combination with metallic components is underestimated. A complicating issue in the galvanic corrosion on aircraft is the sometimes-large distances be-tween the anodic and cathodic areas. The observations suggest that the galvanic influence can cover larger dis-tances than what is generally accepted. The paper addresses research aiming to understand the effect of very large surface area ratios on the galvanic corrosion rate.

INTRODUCTION

The Royal Netherlands Air Force (RNLAF) and Royal Netherlands Navy (RNLN) are using helicopters for a wide variety of naval missions, ranging from counter drugs enforcement, anti-submarine warfare, search and rescue to anti-piracy operations. As a result, the operational environments encountered by the helicopters can be very corrosive. In the past decennia the aluminum Lynx helicopter was used for all these operations. Thanks to a vast experience with aluminum helicopters in general and the Lynx in particular, corrosion was manageable and operational availability of the fleet was assured. From 2010 the NH90 helicopter has replaced the Lynx helicopter. The NH90 is a new generation helicopter that has a full carbon fiber structure. The first operational experiences showed that the metallic parts of the helicopter, like a number of system components, fasteners and gearboxes, experience corrosion. The corrosion is more severe than anticipated based on the experience with the Lynx. This results in an increase in maintenance, maintenance costs and a lower availability of the helicopters.

The severity of the corrosion on the metallic components of the NH90 helicopter cannot be explained solely by the composite structure. It is well known that a large cathodic surface area results in fast corrosion of the anodic metal, especially when the difference in corrosion potential is large like for aluminum and carbon. However, in a helicopter that is most of the time in a (relatively) dry environment the effects of the surface area ratio are expected to be limited. Therefore, to understand the severity of the corrosion an investigation into the contributing factors was started. A number of these factors have been assessed qualitatively and are presented in this paper.

Paper No. 2019-xxxx

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The paper starts with the results of an investigation into the different design requirements that apply to aircraft in general and the NH90 helicopter in particular. The investigation continues with the specific material combination on aircraft and potential differences in behavior between aluminum and composite materials and structures with a focus on the impact thereof on corrosion. The effects of established aircraft maintenance practices in corrosive environments and its effectiveness for composite aircraft is discussed next. Finally, a few examples are shown of excessive corrosion observed on components of a composite helicopter. Similar components have been in service for decades on aluminum helicopters, also in severe corrosive environments, without much corrosion.

CARBON STRUCTURE VERSUS ALUMINUM STRUCTURE

It is well known that carbon fiber reinforced composites are insensitive to corrosion. Therefore, these composite parts do not need any corrosion protection. However, in a corrosive environment the carbon can act as a very noble material. Any metallic component or system that is attached to the carbon structure of the helicopter is less noble than the carbon structure itself and will corrode if it is left without corrosion protection. However, insulation of the carbon composite from the environment can also be beneficial in reducing the corrosion on the metallic parts because the cathodic reaction is prevented. Several specifications provide guidelines and design rules on how to reduce the risk of galvanic corrosion. However, sometimes other requirements have a higher priority for functional reasons so that the corrosion prevention is compromised. In the next paragraphs a number of potentially contradictory requirements are highlighted together with their consequences for corrosion and corrosion prevention.

Lightning protection requirements

In order to understand the high corrosion rate that is observed on the composite helicopter compared to an aluminum helicopter, knowing the differences in design and design requirements is a good starting point. Aircraft structures made from aluminum alloys are generally well protected against lightning strikes. The current is easily dissipated along the fuselage because of the good conductivity of these metals. Conversely as carbon composite material has a lower electrical conductivity it handles lightning strikes poorly. In order to avoid damage to the structure the current dissipation must be improved. Therefore, a copper or bronze wire mesh is embedded in the carbon fiber reinforced plastic (CFRP) and copper-bonding foils connect individual structure panels. A fiberglass ply is laminated on top of the copper mesh and bonding foils in order to isolate the copper parts on the outside. However, since most aircraft systems also require electrical bonding, they are either installed partly on a copper bonding strip or bonding is facilitated with a dedicated bonding wire. Through these bonding foils and wires the aircraft systems also have a galvanic coupling with a very large part of the composite aircraft structure. This galvanic coupling is unfavorable from a corrosion point of view. Another way the systems and components are coupled to the structure of the aircraft is through the fasteners. For attaching components to the structure threaded inserts and riveted nut plates are often used. When the threaded inserts and nut plates are installed in the CFRP structure it is almost impossible to completely isolate them electrically. Even when rivets are wet installed with sealant the close tolerances required for a correct fit combined with the forces involved in the installation process will cause sealant to squeeze out and allow the fiber ends too touch the rivet stem. The threaded inserts are usually glued into position but even though the majority of the insert surface is covered with glue electrical contact is still possible. The relatively tight fit at the top surface makes electrical contact possible through the fiber ends. Measurements performed on the helicopter show that almost all metallic components installed are electrically coupled to the CFRP structure. With all the individual structure elements coupled by bonding foils or copper mesh, the structure of the helicopter can be considered to be one very large cathode. The aircraft system components then can be seen as relatively small anodes. Knowing this makes it even more important to protect the metallic parts from the electrolyte.

Parts design

One of the other differences is in the geometry of carbon composite components and structures. The strength of carbon composite parts and the capability to carry loads is highly dependent on the orientation of the carbon fibers. This often causes parts to have holes and openings that are necessary from a manufacturing point of view. These openings allow in-service air circulation, and thereby the entry of salt and water containing air when the aircraft is flying at relatively low altitude over sea. The salt will deposit inside the composite component and can cause corrosion of metallic fasteners or metallic parts. A detail that supports the development of corrosion is the fact that carbon composite components are often not painted or primed on the inside. The reason for this is that the composite material is insensitive to corrosion and therefore needs no paint. If metallic parts like frames and ribs are attached to the carbon composite, then these metallic parts are painted. The fasteners, however, are usually not

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painted because either stainless steel or titanium fasteners are selected that are assumed to have sufficient corrosion resistance.

System integration

In the aviation industry a lot of generic systems and components are used on different aircraft types. The outside air temperature probe and ice detector probe are examples of such systems. During the integration of these systems, the installation requirements commonly used on aluminum aircraft are used on carbon composite aircraft as well. These installation requirements do not take in account the high risk of galvanic corrosion when installed on a composite structure. Additional installation requirements are necessary to prevent corrosion that causes failure of these components that work flawlessly on aluminum aircraft. These additional installation requirements must be defined by the aircraft OEM, because the systems supplier has no information about the structure of the aircraft on which its components are installed. The information that has to be provided by the systems supplier are the functional requirements for proper operation of the components or systems.

Surface condition

The surface of the metals in contact with the electrolyte is generally covered with a surface protection. This usual-ly is a paint system, a passivation layer or anodizing. This is different for the carbon composite parts. A very thin layer of fiberglass covers large areas of the external parts; the internal parts are either bare or covered with a ply of polyvinylchloride. The condition of these surface protections, especially the porosity, plays an important role.

Thermal properties

Differences in the thermal properties of aluminum and CFRP can play a significant role in the possibility for corro-sion to occur. CFRP is anisotropic, the coefficient of thermal expansion (CTE) in axial direction is almost zero or slightly negative, while the CTE in radial direction can range from 35 to 57 µm/(m⋅°C). Aluminum is isotropic and has a CTE of approximately 24 µm/(m⋅°C). Aircraft are operating in a wide range of temperatures which causes large differences in expansion or contraction between the aluminum components and the CFRP structure they are mounted on. Over time this may lead to cracks in the surface finish and sealant, allowing the electrolyte to enter.

Even though carbon fibers have a relatively high thermal conductivity, the epoxy matrices in fact have not. This results in a poor thermal conductivity for fully cured CFRP compared to aluminum which conducts heat much better. This implies that when for example a cold aircraft is parked inside a warm hangar or when cold aviation fuel is loaded in the internal tanks, condensation will occur preferentially on the aluminum parts. This is unfavora-ble from a corrosion point of view and may increase the risk for corrosion on those parts.

Area and distance effects

Area effects in galvanic corrosion involve the ratio of the surface areas of the cathode and the anode. When the exposed area of the anode is relatively small compared to the cathode under electrolyte-immersed conditions, the corrosion attack on the anode can be very severe. Conversely if the exposed area of the anode is large with re-spect to the exposed area of the cathode the acceleration of corrosion may be negligible. Normally the corrosion will occur near the junction of the anode and cathode due to the effects of the electrolyte resistivity. With large areas of the carbon composite structure being covered with a thin film of sea salt aerosol deposit it is believed that the combined effect of area ratio and distance have a significant effect on the corrosion driving force.

OBSERVATIONS ON COMPOSITE HELICOPTERS

Static electricity and sea salt aerosols

During base maintenance inspections of the helicopters it was noticed that most of the carbon composite parts, have a fine and homogeneously distributed white, powdery film on all the surfaces, internal and external, horizontal and vertical, even on inverted parts. This powdery film was believed to be salt deposit. Such a powdery film has never been observed on an aluminum helicopter.

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Samples of the powder were taken from various parts of the helicopter and chemical analysis with Energy Dispersive X-ray (EDX) analysis showed that the samples contained large amounts of Sodium, Magnesium and Chloride. Carbon composite structures are known for their ability to become electrically charged when simply exposed to airflow. Flight through precipitation increases the charging even more. Unlike metallic aircraft structures, carbon composite structures have difficulties with dissipating the electrical static charge. The build-up of the electrical static charge is believed to play an important role in the accumulation of salt on carbon composite parts, especially under sea salt aerosol conditions. Sea salt aerosols (SSA) are formed by surface winds across the ocean waves or they can be generated by the helicopter itself when hovering at low altitudes above the sea. Dissolved salts like NaCl and MgCl2 are present as electrically charged ions. According their charge the ions may be attracted or repelled by an electrical field. Research is currently ongoing to examine if a static electrically charged CFRP structure can attract salt water aerosols due to the electrostatic attraction (ESA) effect. The research consists of experiments combined with in-flight measurements on the helicopter. Figure 1 shows the experimental setup of an actual NH90 composite part that is electrically charged in a mist of salt water aerosols. It can be seen that the salt water aerosols are attracted by the charged composite component.

Figure 1:

Electrostatic attraction of salt water aerosols.

Research performed by Schindleholz [2] raises the question whether surfaces contaminated by SSA ever dry in natural ambient conditions. MgCl2 is highly hygroscopic and dries only at relative humidities as low as 11%. Salt deposits, such as SSA deposits, that contain water can initiate and sustain corrosion. Knowing this makes the research into the deposition of SSA on the CFRP structure very relevant because it can play a major role in the area and distance effects of galvanic corrosion.

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Corrosion rate

The corrosion rate that is observed on the carbon composite helicopter is much higher than the corrosion rates observed on aluminum helicopters that operate in similar environment. This can be attributed to the localized type of the corrosion found, but the galvanic coupling plays a role too. It is hypothesized that the surface area ratio of the noble and corroding parts plays a major role in accelerating the corrosion. The whole aircraft needs to be connected to enable the dissipation of electricity in the case of lightning strike. Components need to be coupled to the structure for proper functioning and to eliminate static noise due to electrical static charges. A result of these coupling requirements is that each component is galvanically coupled to the whole CFRP structure. In the case of corrosion this implies that the surface area of the cathode is larger than the surface area of the anode by a much larger factor than what is generally taken into account in experiments and hence the acceleration of the corrosion caused by the surface ratio is much larger [1]. This surface area ratio is even further increased by the surface protection that is usually not present on the inside of CFRP structures. Metallic parts are almost always protected by a paint system, anodizing or passivation and corrosion occurs when this protection is damaged. Then a very small part of the metal is exposed and the galvanic coupling to the cathodic CFRP structure results in a very large surface ratio and corresponding high corrosion rate.

Shift of corrosion

It has been observed that protection of a corroding part leads to a shift of the corrosion. The next least-noble part will become the anode and will start to corrode. It is not always possible to predict which part this will be, because often the corrosion takes place at a location where the protection is damaged. The observation that the corrosion shifts to another part upon protection of the initially corroding part of a component teaches us that we should not aim at protecting all components. Damage to the protection scheme will occur sooner or later, and hence corrosion cannot be prevented fully. The aim of a corrosion protection program, therefore, should be to protect those parts that are most expensive or critical to the operational availability. Additionally, the corrosion must be relatively easy to find during an inspection.

TYPES OF CORROSION FOUND

Several types of corrosion can be found on aircraft in general and composite aircraft in particular. The most common types of corrosion found on the composite helicopter are crevice corrosion and pitting corrosion on aluminium as well as on (stainless) steel. Filiform corrosion near panel edges and fastener holes is also observed frequently. The corrosion is often accelerated by galvanic coupling to the carbon composite structure.

A result of the frequent occurrence of crevice corrosion is that corrosion inspections require attention to detail, because the signs of corrosion are easily overlooked. The corrosion is hidden in the crevice, and only revealed by minor traces of corrosion product or cracks in the protection such as sealant or paint. When these small indications are found the crevice corrosion can be massive.

Crevice corrosion is much more severe on carbon composite aircraft than on aluminum aircraft because the structure of the aircraft is very noble. Salt water can enter a split line when the paint or sealant protecting it is cracked. Then crevice corrosion can start if the driving force is sufficiently high to break down the additional protection of the mating surfaces. In a carbon composite – aluminum joint this driving force is high, while the driving force is low in an aluminum – aluminum joint. As a result, the crevice corrosion will initiate easily in the carbon composite – aluminum joint as compared to a joint of two aluminum surfaces.

Where stainless steel components perform well on aluminum aircraft, they may suffer from widespread pitting corrosion on a carbon composite aircraft. The pitting corrosion can be found underneath salt crystals that are deposited on the stainless-steel parts. These salt deposits form when the aircraft has flown through salt-containing air and salt water or even salt crystals deposit on the surfaces. These salt deposits attract water when the relative humidity is high(er), which results locally in a highly corrosive environment and the initiation of pitting corrosion.

Selective leaching or dealloying corrosion is found on the rivets of the fire shields. The fire shields are mounted on to the carbon composite structure and are one of the few locations were solid rivets are used. Green discoloration of the rivet heads and ends has been observed. Although the corrosion process seems to proceed slowly, this will eventually lead to a drastic change in mechanical strength of the rivets.

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CORROSION EXAMPLES

The corrosion examples below are components that have been on a helicopter for less than 600 flight hours. The helicopters have been flying in saline environment, part of the time embarked on a ship. These examples serve to illustrate the fast corrosion on the composite helicopter when used in saline environment.

Antennas

Antennas require a good electrical bonding to the structure in order to function properly. Therefore, the contact surface of the antenna is bare aluminum with a conductive conversion coating to protect it against corrosion. On the aircraft side copper bonding strips are laminated on the structure to provide good electrical bonding, because the conductivity of the carbon composite is low. When the antenna is mounted into position a galvanic coupling between the carbon composite and the aluminum is made through the copper bonding strip. A sealant layer along the antenna circumference provides water tightness of the installation.

Figure 2:

Corrosion of antenna mount at the interface with the structure.

Figure 3:

Cracking of the sealant due to corrosion at the interface.

Due to vibrations, differences in CTE and aging of the sealant, small cracks appear which compromise the water tightness. It is also possible that water can enter from the inside or via the attachment screws, if these are not sealed on inside and outside. When water can enter between antenna and structure, then corrosion can develop and progress very fast due to the crevice geometry in combination with a large galvanic couple (Figure 2). The corrosion becomes visible only when the corrosion products result in such stress built-up that the split line opens or the sealant shows severe cracking (Figure 3).

Some antennas have split lines that are not in direct contact with the structure of the helicopter, but still show much more severe corrosion on the carbon composite helicopter than what is usually found with similar antennas on aluminum helicopters (Figure 4). Here the galvanic coupling of the antenna to the carbon composite structure, even if it is at a larger distance, is believed to play an important role.

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Figure 4:

Corrosion of antenna at a split line.

Bonding foil

Bonding between the carbon composite structure and aluminum frames is achieved using bonding foils. These are copper foils that are connected to the aluminum frame with rivets and conducting paste. The circumference is sealed to prevent water entry. The sealant and bonding paste will develop cracks over time due to ageing, flight loads, and more importantly due to the large difference in thermal expansion between the paste, aluminum frame, copper strip and carbon composite skin. Water can enter the bonding foil area once the sealant is cracked and corrosion will develop at a high rate because of the galvanic coupling in combination with the confined area (crevice corrosion). Figure 5 illustrates the amount of material removal required for corrosion repair at a bonding foil location.

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Figure 5:

Bonding area after corrosion removal on the aluminum frame under a bonding foil.

Complex geometries

Aircraft systems and components often have complex shapes and geometries which make it difficult to apply a fully closed layer of protection on them, especially at sharp edges and corners. Therefore, it is important to select a suitable corrosion preventive compound that has the ability to maintain a closed layer even on complex shapes and geometries. When testing several CPC’s big differences were found between them. The influence of the climatic conditions and the general wear and tear during flight degrades the CPC’s over time. This degradation is generally first noticed on the sharp edges and corners, as illustrated in Figure 6.

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Figure 6:

Corrosion at sharp edges due to premature deterioration of the corrosion preventive compound.

AIRCRAFT MAINTENANCE IN CORROSIVE ENVIRONMENT

Fresh water rinse and washing

Daily fresh water rinsing and periodic desalination washing are a common part of the maintenance procedures when operating aircraft in corrosive environments. These processes are effective only on those parts of the aircraft that can be flushed. In most cases these are the external parts, although for some aircraft it is required to open some of the cowlings to flush the inside area as well (after protecting the electrical components that cannot be flushed). Many areas, both interior and semi-exterior, are aerated, but cannot be accessed easily for cleaning or washing. Salt that is deposited in these areas and will remain on the components and systems. As a result, these components are vulnerable for corrosion. Tests are ongoing to determine if running rotor rinsing is beneficial for those areas that cannot be reached by normal washing. With running rotor rinsing water is sprayed onto the running rotors to create a very fine fresh water mist around the helicopter that is intended to access those areas that cannot be reached by regular washing.

Application of CPCs

The structure of aluminum aircraft is protected against corrosion after washing or rinsing by spraying with a water-displacing corrosion preventive compound (CPC). These corrosion preventive compounds often contain a corrosion inhibitor to add in the corrosion protection. This is proven to be very effective in reducing the corrosion, because the water is displaced from crevices and corrosion inhibitors are introduced into these crevices. On carbon composite aircraft the spraying with a CPC is not allowed, because the solvents in the CPC can degrade the matrix material of the carbon composite. Over time this may compromise the structural integrity of the aircraft.

A result of the above is that the corrosion preventive maintenance that is good practice on aluminum helicopters cannot be applied on composite helicopters. The daily rinsing and desalination wash clean the carbon composite exterior of the aircraft that is not sensitive to corrosion. The corroding components cannot be rinsed and water cannot be displaced from crevices. This results in a higher exposure to corrosive conditions of components on a composite aircraft as compared to an aluminum aircraft.

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Corrosion inspections

Additional to the cleaning and application of CPCs are the corrosion inspections. Those parts that are least noble have the highest risk of corrosion and should be inspected. On aluminum aircraft corrosion occurs often on the structure, and much less on the systems because the materials in the systems are more noble and hence less sensitive to corrosion. On carbon composite aircraft, on the other hand, the structure itself is the most noble part and all systems are less noble. Those parts of the systems that are made of the least noble material will corrode.

The corrosion inspections that are performed are directed towards the components or aircraft parts that are expected to corrode. Based on the vast experience with aluminum helicopters, the focus of corrosion inspections is on the structure. If components and systems are subjected to a corrosion inspection the directions are a general corrosion inspection. Given the types of corrosion found on the carbon composite helicopter and the small signs of corrosion that can be observed, more specific directions of where to look for corrosion would help in finding the corrosion. Additionally, the maximum corrosive attack that is allowed on components is usually defined in a generic way instead of based on the detailed requirements of the components. As a result many components are rejected owing to a too large corrosive attack, while the functionality of the component is not hampered.

Corrosion repair

Traditionally, corrosion repair is a task for a structure engineer since the corrosion is usually found on the structural parts of the aircraft. However, on aircraft with carbon composite structure the structure itself does not suffer from corrosion but the metallic parts of the different aircraft systems do. As a result the corrosion repair is much more complex because it involves the system engineers for each of the different systems involved. This is especially complex for parts that are obtained from vendor suppliers. An additional complexity is added by the fact that corrosion occurs in many cases on the interface between the carbon composite structure and the mounting flanges of the aircraft systems and components. For these cases the aircraft OEM and the vendor supplier need to agree on the measures to be taken to prevent corrosion.

At present the rejection criteria for corrosion on components and systems are defined on a case by case basis after corrosion is found and reported, because the large-scale corrosion of components is unknown for aluminum aircraft. This is not only time consuming and a large burden on the engineering resources of the industry; it also leads to a longer downtime of the aircraft.

CONCLUSIONS

A corrosion free aircraft is an illusion; eventually at some point corrosion will occur. Therefore, it is key to design a corrosion protection maintenance program that will divert the corrosion to the cheaper, easy to replace and easy to inspect parts of the aircraft or aircraft system. For example, selected bolts and nuts can be left unprotected on purpose so they can act as a sacrificial part in a system. By doing this, the expensive and important parts of the aircraft system can be saved from corrosion. During the regular maintenance inspections these sacrificial parts are replaced before the corrosion compromises the function of that specific part. This also improves safety because the location of the corrosion can be found very easily.

On aluminum aircraft severe corrosion damage to the system components is not expected and therefore not specifically taken into account when the inspections are formulated. The check for corrosion is often part of a visual condition inspection. The limitations and criteria given for corrosion, if any, are very generic. When these components are used on an aircraft with a carbon composite structure the approach towards the inspections must be different. Experience after (base) maintenance shows that there is not only a need for detailed inspection criteria, but also for dedicated rework and repair procedures for the components and systems.

The corrosion protection needs to be applied and maintained perfectly and without any flaws. Even the smallest crack or gap in the paint finish, sealant or corrosion preventive compound can lead to (salt) water ingress. When this happens corrosion can develop very rapidly due to the large difference in galvanic potential between the carbon composite and the metals used. Maintenance personnel and certifying staff involved in the maintenance process should be made aware of this requirement. This is not only true for the application process of the different corrosion preventive measures used, but also for the condition checks that are part of the corrosion inspections.

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The corrosion issues on a carbon composite aircraft are different from those on aluminum aircraft. Therefore, the requirements for personnel responsible for the corrosion repair are different. Where a structures specialist is needed for the corrosion repair on an aluminum aircraft, a systems (design) engineer is required for solving the corrosion on a carbon composite aircraft.

REFERENCES

1. Palani, Siva. Modelling of galvanic corrosion on hybrid structures in aircraft. October 2013,PhD thesis, Re-search Group Electrochemical and Surface Engineering, Free University of Brussels, Belgium

2. Schindelholz, Eric J. Towards understanding surface wetness and corrosion response of mild steel in ma-rine atmospheres. May 2014,PhD thesis, School of Engineering and Applied Sciences, University of Virginia, Virginia, USA.