Advances in protective coatings

and processes for aerospace

applications
Chris J.E. Smith

Introduction been extensively used for cold and priority. A survey of the procedures used
The aerospace industry relies heavily vapour degreasing because of their non- on the Black Hawk Helicopter has
on protective treatments and processes flammability and low toxicity[1]. shown that over 40 per cent of the
to ensure that the structural integrity of The depletion of the ozone layer is maintenance operations involve the use
an aircraft is not degraded in service as thought to be due partly to the rise in of hand-wipe cleaning/degreasing and
a result of operating under harsh chlorine levels in the upper atmo- vapour degreasing. Both 1,1,1-trich-
corrosive conditions. Many of the sphere[2]. It is believed that chemicals loroethane and 1, 1, 2-trichlorotrifluoroe-
chemicals and processes currently such as 1,1,1-trichloroethane make a thane have both been used for this
employed in metal finishing have been significant chlorine contribution to the purpose. The use of isopropyl alcohol
found to cause pollution and long-term atmosphere. The results of a study for removal of dust and sanding grit is
damage to the environment. Legislation carried out by the American Airlines being recommended and aqueous
and international agreements are now in Tulsa Maintenance and Engineering cleaners for the general cleaning of
place which ultimately will lead to a Center to eliminate trichloroethane grease, oil and dirt. For heavy deposits
ban or major reduction in the use of vapour degreasing have recently been of grease N-methyl pyrrolidone is to be
many of these processes and coatings. outlined[3]. The study looked initially at employed.
The aircraft constructors and operators reducing the volume of trichloroethane
are seeking to adopt new protective consumed.
schemes and treatments which will A programme of waste minimization Pretreatment and paints
satisfy future environmental require- was implemented through the reduction The bulk of the structure on modern
ments. in the number of vapour degreasers in military and civil aircraft continues to be
At present there are a number of use, the upgrading of equipment to meet manufactured from heat-treatable
areas which are of concern to the air- regulatory requirements, improved aluminium alloys. These materials are
craft industry. These include the use of operator training and the recycling of particularly susceptible to localized
chlorinated solvents for cleaning, solvents. Alternative processes and corrosion when exposed to chloride-
ebromate-based corrosion inhibitors, chemicals such as aqueous cleaning to containing environments. Protective
volatile organic compounds (VOCs) as replace solvent vapour degreasing and treatments are therefore employed on all
solvents and thinners in paints and hydrocarbon and terpene cleaners for aerospace components and involve the
heavy metal coatings such as cadmium cold cleaning and wipe-on applications use of pretreatments such as chromate
plating. This article outlines the were also adopted. As a result of the filming and chromic acid anodizing and
advances which are being made USA's decision in 1992 to accelerate the the application of paint primers
towards the development of new phasing out of trichloroethane, containing leachable chromate-based
protective treatments and processes American Airlines is reconsidering inhibitors. Two environmental issues are
which will satisfy future environmental vapour degreasing processes using of immediate concern; the use of
legislation. perchloroethylene. chromates and the use of high volumes
In a review of new aircraft finishes, of VOCs.
the progress being made by the Boeing
Surface preparation and Commercial Airplanes to replace vapour
cleaning degreasing solvent substitutes with Pretreatments
Chlorinated solvents have been widely heavy duty emulsion cleaners has been The pretreatments used are generally
employed for the degreasing and described[4]. It is intended to use anodizing and conversion coatings.
cleaning of metal surfaces prior to metal emulsion cleaners instead of vapour Their main purpose is to promote paint
coating or painting. Chemicals such as degreasers for sheet metal in some of its adhesion but additionally they give
1, 1.2-trichlorotrifluoroethane and methyl major finishing lines. some corrosion protection to the alloy.
chloroform (1, 1, 1-trichloroethane) have The US Army has put into place a In the anodizing process, the natural
programme to eliminate hazardous and oxide present on the surface of alumi-
environmentally acceptable materials nium is thickened by an electrolytic
© British Crown Copyright 1995/DRA.
Published with the permission of the from the maintenance of aircraft[5]. The process. The oxide film produced
Controller of Her Britannic Majesty's elimination of ozone-depleting com- consists of two layers; a thin, dense
Stationery Office. pounds has been identified as a first barrier layer at the metal surface and a

Aircraft Engineering and Aerospace Technology, Vol. 67 No. 5, 1995, pp. 13-16, MCB University Press, 0002-2667 AEAT September/October, 1995 13
 thicker, porous outer layer. The pore size primer. The dry paint film contains at effective when applied to a chromic acid
and coating thickness are dependent on least 15 per cent by weight of strontium anodized surface. This was further
the bath temperature and the applied chromate pigment which will slowly confirmed when samples were evaluated
voltage. leach from the paint in the presence of using a crevice corrosion test.
For aerospace applications anodizing moisture. Chromates are very efficient One approach which has been pro-
is usually carried out in either chromic in inhibiting corrosion on aluminium posed is to consider the phasing in of
acid or sulphuric acid electrolytes. The alloys, particularly in the presence of chromate-free primers for selected
preference in the UK is for chromic acid chlorides. Even at very low concen- surfaces and areas of an aircraft. This
anodizing, which tends to produce trations (~0.01 mM/1), chromates are still philosophy has been outlined by
thinner films than sulphuric acid effective at inhibiting corrosion[8]. Gerrits[11]. Components manufactured
anodizing and has less damaging effects Experiments at the Defence Research from composite materials are generally
on fatigue properties. Agency (Farnborough) suggest that treated with a standard chromate-
The anodic film acts as a protective during exposure to moisture this is the pigmented primer. Since these materials
barrier, isolating the metal substrate concentration of chromate likely to be are not susceptible to corrosive attack, a
from the environment. The effectiveness leached out of the primer. non-chromate primer could be
of this barrier may be improved by One of the main concerns has been substituted fairly readily. The protection
sealing the pores in the anodic film. This the carcinogenic nature of chromates of exterior surfaces has also been
can be achieved most simply by and there has been considerable effort to considered[11]. A scheme based on the
immersing the anodized part in de- find alternative corrosion-inhibiting use of a filiform corrosion-resistant
ionized water at 96°C or above for a pigments. Several types of compounds, primer, overcoated with a chromate-free
minimum time of ten minutes. Other including salts of mercapto-carboxyllic intermediate primer and an ultraviolet-
sealing processes are available which acids and cerium salts, have been resistant polyurethane top coat has been
incorporate chromates into the pores. identified which will inhibit aluminium proposed.
Opinions differ on the use of sealed corrosion, but these are only effective
anodic films if the component is to be when present in concentrations rather
Low VOC paints
painted. Normal UK practice is to paint higher than those identified for chro-
In addition to concern about the use of
directly onto the unsealed film within 16 mates[9].
materials containing chromates, there is
hours of anodizing, while in North
increasing pressure to reduce the level of
America the tendency is to paint on to
solvent emission from paints. One
sealed films.
approach is to carry out the painting in a
The aerospace industry is currently
spray booth large enough to house the
looking at alternatives to chromic acid
anodizing. The Boeing Aircraft Com-
"Primers complete aircraft. Facilities planned for
the US Air Force have recently been
pany has indicated that phosphoric acid described[12]. Air in the booth will be
anodizing and boric-sulphuric acid containing various circulated through carbon filters which
anodizing may have applications as will remove the VOCs. Efforts are being
pretreatments for painting[4].
Alternative pretreatments to anodiz- non-chromate made by the manufacturers to produce
low VOC, high solid materials suitable
ing are conversion coatings. A number for aircraft applications. Target levels
of these are available commercially and inhibitors have been set which will reduce the level
those used on aluminium alloys nor- of VOCs in paints and varnishes
mally contain chromic and phosphoric between 45 and 60 per cent.
acids. The process involves either are not as Although electropainting, water-
immersing the component in a bath
based paint systems and powder coat-
containing the solution or brushing or
swabbing the solution onto the compo-
effective" ings have found many applications
outside the aircraft industry, they have
nent surface for a few minutes. The
not been used to any great extent on
films produced are relatively thin (1- aerospace components. The devel-
5µm) and are complex in composition. opment of a water-borne epoxy primer
Research is in progress to develop has been described[13,I4]. It is pro-
Trials made on primers containing
chromate-free conversion coatings. duced as a two-component high solids,
various non-chromate inhibitors suggest
Cohen has recently reviewed replace- solvent-borne material. One component
that while they give some protection to
ments for chromium pretreatments on contains the pigmented epoxide resin
aluminium alloys they are not as effec-
aluminium[6]. Some progress is being solution and the other contains a
tive as the standard chromate-pigmented
made in the development of molybdate polyamide curing agent solution.
primers. Matz has presented the results
and phosphate based coatings. A surface
of laboratory tests carried out to Prior to application, the two compo-
modification process using cerium
compare the corrosion performance of nents are mixed and water is added,
based solutions has also recently been
chromate-free primers with a standard forming an emulsion. On application
described[7].
chromate-pigmented primer[10]. Four and cure, the coating has similar chemi-
primers supplied by leading aircraft cal and physical characteristics to
Chromate-free primers paint manufacturers were evaluated standard aerospace epoxy primers. The
Following the application of a suitable using a filiform corrosion test. The main difference is that they contain
pretreatment, the surface of the results obtained showed that the barium rather than strontium chromate
component is painted with an epoxy chromate-free materials were only corrosion-inhibiting pigments.

14 Aircraft Engineering and Aerospace Technology

Metal coatings Apart from the corrosion behaviour of civil and military aircraft has always
Metal coatings are employed on aircraft the coatings and their compatibility with been a major problem. The polyurethane
structures to improve the corrosion aerospace aluminium alloys, research materials are highly resistant to fluid
resistance and wear properties of into the effects of coatings on the fatigue attack and phenolic and methylene
components and ensure that there is strength of steels and the frictional chloride-based paint strippers must be
galvanic compatibility between parts properties of the coatings is also being employed. These chemicals are toxic
manufactured from different materials. carried out. and very unpleasant to handle and their
For many years, cadmium plating has use is becoming more restricted due to
been the preferred protective coating for environmental and occupational
use on steels. Cadmium plating regulations. Two approaches have been
possesses good barrier corrosion-
resistant properties and is also a
"The performance employed; the development of novel
blasting techniques and the introduction
sacrificial coating. It is galvanically of selectively strippable paint schemes.
compatible with aluminium alloys so of non-passivated Plastic media stripping has been
that the risk of dissimilar metal cor- adopted by a number of operators for the
rosion is minimized.
Another property which is important zinc-nickel removal of paint from the exterior
surfaces of aircraft although there
when cadmium plating is applied to steel remain some reservations about its use
fasteners is its high natural lubricity.
This gives reproducible torque - tension
coatings on clad materials, thin sheet and carbon
fibre composites. More recently the use
properties. The main disadvantage of of wheat starch media for the stripping
cadmium is that its compounds and may be of paint from both metallic and com-
corrosion products are classified as
posite materials has been des-
harmful or toxic. Within the European
Union restrictions have been placed on inadequate" cribed[21,22]. Other blasting techniques
which are either under development or
the use of cadmium plating for general
engineering purposes and in industries which are being assessed for aerospace
such as food production and in the uses include blasting with ice particles,
manufacture of household goods. Basic research into the corrosion dry ice and bicarbonate of soda[22].
Certain industries, for example behaviour of electro-deposited zinc- Guidelines for the evaluation of aircraft
aerospace and mining, are able to nickel coatings has been undertaken at paint-stripping materials and processes
continue to use cadmium plating where the Structural Materials Centre at are currently being prepared by IATA.
acceptable alternatives have not so far DRA(Farnborough). The effect of nickel Selectively strippable paint schemes
been identified. Research into replace- content on the balance between the have been developed for use on the
ments for cadmium plating for use on barrier and sacrificial properties of the external surfaces of aircraft[23,24].
aerospace parts is being undertaken in coatings has been investigated[18]. The Several approaches have been employed
Europe and North America. maximum time to red rust when electro- including the use of intermediate coats
plated panels are exposed to 3.5 per cent which are applied over the primer before
The most promising coatings are neutral salt fog occurs when the coating
either aluminium or zinc based[15]. Ion final painting with a polyurethane finish.
contains 14 per cent nickel. This has The results of trials conducted on a
vapour deposited (IVD) aluminium
been shown to represent a balance British Airways Concorde have been
coatings were introduced some years
between the barrier and sacrificial described[23]. Various non-methylene
ago by the McDonnell Aircraft Com-
properties of the coating. Recent chloride-based strippers were
pany as an alternative to cadmium
research suggests that the performance
plating for aerospace parts and fasteners successfully used to remove the
manufactured from steel[16]. More of non-passivated zinc-nickel coatings
polyurethane top coat from the aircraft
recently the Boeing Aircraft Company exposed in a marine atmosphere may be
while leaving the intermediate coat and
has described the development of inadequate[19].
primer in place.
electro-deposited zinc-nickel coatings In collaboration with Salford
for aerospace applications[17]. In University, the DRA has undertaken
Europe the IVD aluminium coatings research into unbalanced magnetron Repair of protective coatings
have so far failed to gain general sputtered aluminium-magnesium coat- Brush plating and anodizing treatments
acceptance for aerospace purposes. ings. Recent work indicates that coatings are used in the general engineering field
A Garteur collaborative research containing 20 per cent magnesium give for the in situ repair of metal coatings
programme is currently examining over 800 hours protection in neutral salt and anodic films. Examples of the use of
a range of cadmium replacement coat- spray compared with 420 hours for cadmium brush plating on aircraft
ings including electro-deposited zinc- electrodeposited cadmium[20]. Research components include the repair of local
nickel and zinc-cobalt-iron alloy coat- into these coatings for the protection of damage on landing gear components,
ings, metallic ceramic coatings, aerospace fasteners is continuing. replacing eroded cadmium on ultra high
electrodeposited aluminium, unbalanced strength fasteners on aircraft skins and
magnetron sputtered aluminium- the touching up of defective areas on
magnesium coatings and IVD alumi- flap tracks[25]. With the pressure to
nium. Eight organizations are partici- Maintenance and repair replace cadmium plating it will be
pating in the programme; Aerospatiale, processes necessary to develop suitable, non-
British Aerospace, Defence Research Paint removal cadmium repair schemes.
Agency, Deutsche Aerospace, Fokker, The stripping of polyurethane paint The situation is similar with brush
NLR, Saab and Shorts. schemes from the external surfaces of anodizing. A brush chromic acid anodiz-

AEAT September/October, 1995 15

 ing process has been employed for the 9. Smith, C.J.E., Baldwin, K.R., Metal Finishing Forum, San
repair of damaged coatings on aircraft Hewins, M.A.H. and Gibson, M.C., Antonio, Texas, Paper No.830686,
skins[26]. New, chromate-free treat- "A study into the corrosion Society of Automotive Engineers,
ments will clearly be required in the inhibition of an aluminium alloy by Inc., Warrendale, PA, 1983.
cerium salts", in Costa, J.M. and 18. Baldwin, K.R., Robinson, M.J. and
future.
Mercer, A.D. (Eds), Progress in the Smith, C.J.E., "The corrosion resis-
Understanding and Prevention of tance of electrodeposited zinc nickel
Corrosion, Institute of Materials, alloy coatings", Corrosion Science,
Conclusions London, 1993, pp. 1652-63. Vol. 35 Nos 5-8, 1993, pp. 1267-72.
Many of the chemicals and processes 10. Matz, C.W., "Chromate-free 19. Baldwin, K.R. and Smith, C.J.E.,
used in the protection of aerospace corrosion inhibiting primers, state of "The corrosion behaviour of zinc
structures are now considered to be the art and prospects", 3rd
alloy and aluminium alloy coated
hazardous and damaging to the environ- International Aerospace Corrosion
steel panels in a marine
ment. Worldwide efforts are being made Control Symposium, Paper 9, SP
environment", Corrosion Reviews,
Conferences, Gravesend, 1994.
to develop new protective treatments Vol. 12 Nos 1-2, 1994, pp. 141-55.
and schemes which will satisfy future 11. Gerrits, J.C.M., "The green revolu-
20. Baldwin, K.R., Bates, R.I., Arnell,
environmental legislation. tion", 2nd International Aerospace
R.D. and Smith, C.J.E.,
Corrosion Control Symposium,
"Aluminium-magnesium alloys as
Paper 16, Sawell, London, 1992.
corrosion resistant coatings for
12. "Air force spray booths capture steel" (forthcoming).
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16 Aircraft Engineering and Aerospace Technology