Symposium on Priming and Finishing of Nonferrous Surfaces Preaented before t h e Division of Paint and Varnish Chemiatry a t the 84th Meeting of the American Chemical Society, Denver, Colo., dugust 22 to 2 6 , 1932.
Aircraft Finishing $1. R. WHITMORE, U. S. Army Air Corps, Dayton, Ohio
The frameuiork and surfaces of A r m y aircraft are painted to obtain a characteristic color scheme and as a protection against corrosion which m a y cause tremendous changes in the physical properties of the metals. The mailable protective coatings m a y be diaided into three groups: organic coatings (paints, lacquers, greases), metallic coatings (electrodeposits, galvanizing, etc.), and chemical or electrochemical treatments (anodic treatment, Parkerizing, etc.). Each type is used by the A i r Corps, and in practically all cases a paint coating is applied to the metal, preriously treated by one of the other
T
HE framework and surfaces of Army aircraft,arepainted
for two reasons: to obtain a characteristic color scheme, and what is of far more importance, as a protection against corrosion. They are not painted merely because the red rust of iron or the white rust of aluminum are particularly unsightly, but rather because the t,remendous changes in the physical properties of the metals occasioned by this rust make protective coatings for aircraft imperative. Airplanes must be strong, rigid, and light. These requirements necessitate the employment of light strong alloys and an extensive use of thin-gage material in their manufacture. Recently, the metal wing covering of an airplane was examined which had been in service from February, 1928, to January, 1932. The airplane had been stationed a t Dayton, Ohio, and Rantoul, Ill., for the entire 4 years. This midwestern area represents mild conditions from the standpoint of corrosion. The wing covering was corrugated heat-treated aluminum alloy (duralumin), the interior (unexposed) surface of which was coated with spar varnish a t manufacture, whereas the exterior had no protective coating of any kind. This metal covering originally had an ultimate strength of 60,000 to 65,000 pounds per square inch (4218 to 4570 kg. per sq. cm.) and an elongation in 2 inches ( 5 cm.:) of 18 per cent. Table I gives the physical properties obtained on specimens cut from the metal covering and shows clearly the deterioration, especially as regards elongation, after 4 years of service under extremely mild conditions. Surface corrosion of the r i n g covering was barely noticeable, and under magnification Dhe corroded areas were minute. Nevertheless, it was sufficient on this thin-gage metal to reduce +.hephysical properties to a marked degree. Unfortunately, t,he condition is aggravated in this type of alloy, as the corrosion is not so much the surface or pitting type, but one that proceeds along the grain boundaries (intergranular
methods. Practically all steel, brass, and bronze parts and surfaces are cadmium plated, and all aluminum and aluminum alloy parts are anodically treated. Corrosion and vibration tests show that paint coatings f o r aircraft should possess, besides durability, the following characteristics: high resistance to permeability by water, pigments which will tend to passivate metals and preceni! corrosion, absence of pigments of alkaline reaction, good adhesion to metal which m a y not be chemically clean, and high resistance to impact and bending even after exposure. corrosion) and destroys the strength of the metal with little or no surface evidence. When corrosion can progress to this extent under mild conditions of service, it is apparent that protective coverings must be employed when most of the airplanes are operating in tropical and coastal areas where high humidity and salt water are always present. TABLE1.
PHYSICAL PROPERTIES O F METALW I N Q COVERINQ AFTER 4 YEARS
THICKNESS LOCATION OF METAL OF SPECIMEN
Inch 0.010 0.010
of of of of of
wing wing wing wbg wing
ULTIMATI STRENQTH Lb sq zn (kg.j/sq.c,:) 53,000 (3726) 50,000 (3515) 46,000 (3234) 50,000 (3515) 38,000 (2671)
ELONQATION
% ' in E inches
0.010 0.010 0.010
Bottom Bottom Bottom Bottom Bottom
0.013 0.013 0.013 0.013 0.013
Top Top Top Top Top
of of of of of
wing wing wing wing wing
52,000 47,000 50,000 52,000 65,000
(3656) (3304) (3515) (3656) (3867)
6.0 2.0 3.5 5.0 5.5
0.020 0.020 0.020 0.020
Top Top Top Top
of of of of
wing u-ing wing
58,000 (4077) 57,000 (4007) 55,000 (3867) 57,000 (4007)
12.0 8.5 8.0 8.0
wing
6.5 5.5 6.0
...
3.3
PROTECTIOK AG.4IKST PERMEABILITY BY Iv.4TER
The available protective coatings may be divided into three groups: organic coatings (paints, lacquers, greases), metallic coatings (electrodeposits, galvanizing etc.), and chemical or electrochemical treatments (anodic treatment, Parkerizing, etc.). Each type is used by the Air Corps, and in practically all cases a paint coating is applied to the metal previously treated by one of the other methods. Practically all steel, brass, and bronze parts and surfaces are cadmium plated, and all aluminum and aluminum alloy parts are anodically treated. 19
20
I N I)1; S'i' Ii I h I,
A N 1)
E N 1; I N F: I< R I 3 G C ti E M IS?' R Y
Vol. 23. No. I
Tiis treatnient (usually termed "anodic treatment") consisis in inakiiig the aluniinuiri part the anode in a 3 per cent solution of chromic acid at a temimature of 40" C. The voltage across tlie Ijath (the iron tank is made the cathode) is gradually raised from 0 to 40 volts iu 13 minutes. This potential is mninta.ineci fur 35 inisiiiirs wlim the voltage is gradually increased to 50 in 5 minutes and held a t that voltage for 3 minutes. This trentiricnt. plnccs ii bard, adherent,
7
Pyroxylin
12
I'yiuxyiin I'yroxylin
7 id
.-
coatiug of alurrriiiiim oxide on tlir nietal. ' l h coating mconductor of electricity, lias a tiiiakness ~ , fabout 0.55 micron (3 X 1 0 iiiclies), arid absorbs approximately 0.041 gram (f) o f clirmiic acid jrcr sqiinn: m c t w . 'i%e coating offers considcralde pi,tection in itsdf against c~nrosioiiani1 lias tlie added Irature raking a11 (!xcck?nt base for paint ridpoint iif adliesioir and corrosion inat,erials hotlr irom tlii resistance. A paint coating over anodizrd aliiiiiinom :tlloy !rill u'ithstand about five tililc8 as long an exposlire to suit. water as the same coating over uiit,reitt,od inetd. 'F:il~lc 11 gives a few of tlie results obtaincd by upiiig variiriis nretal primers over treated and untreated iiuraluiiiin sheet. The sulfuric acid aiiodic treat,iiieiit coiisisted iii iiiakirig tho panel tile aiiode in 15 per cent by voluriie solation of siilfiiric acid. Sheet lead was used as the catliodc. Tlic treatioent was for 30 minutes with the solutiim niairrtained at 25" C. The wlta,ge across the bath TYBS lirtld constslit :it 12, arid the :tvcrage current dansity was 8.6 ampcrrs per square: foot, (0.93 :tmperes per nq. clrn.). An a1k:iline chromate s d i i t b n was used for tire ininiersiun process. T l i e tiirre of treatment was imini:ks, and teinpernture of tlic wliit~ioii$15' C. Primers 1, 2 , and 3 (Table 11) were products wlrich liad previously iieeri tested arid sltoived p u r , fair, and g o d resistnnce, respectively, tn salt mter expowre. The finish in all cases consisted of two light spray coats of primer. The exposure was rnade by subjecting tlir panels to intcrmitt.ent immersion in a 20 per cent by weight solution of sodium chloride. The apparatus used is stionn in Yigure 1. With this machine t.he specimens are immersed in the salt wat,er for one minute and then raised out of tlie solution and allowed to airdry for 15 niinutes before the next immersion. The machine operates automatically and continuously. Ha.rd robber tanks arc used as containers for the salt solution.
I'yiiiayiin
None CrOi, anodic:
xonr CrO,. Rnodlc
7 29 6 29
c
c
c A
As previously stalcd, s t d , I m s s , and Imxm par& an' cadmium plated as the initial protective coating. Cadmiuiii offers several desirable features as a protective coating for aircraft parts. It lias exeslleiit rcsistarice to salt water; only a tliin (0.0003 t o 0.000,5 inc.11, or 0.0008 t o 0.0013 em.) coating is necessary for satisiactory protection as compared to 0.0015 to 0.0W25 iiicli (0.0038 to O.OO(i4 em.) for equivalent protcction by zinc; it is electronegative t,o the alnrriinimi al-
.4. B.
c.
llost-tieeted
