L S$aff-IndusdrgCollaborative Repo-’ M. J. PRIGOTSKY
MERRrw L’ USTENS Associate Editor
T .
in collaboration with
HE action of chromate ion in protecting metal surfaces against common has been known and commercially exploited in various surface p&vating pigments for a number of years (IS). The w of paint primer coats containing soluble chromate is an obvious m e a of utilizing this effect, but far many years the problem of economic production of a chromate pigment with properly controUed solubility and other desirable pigmentary pmpertiea proved to be extremely di5cult. The mom wmmon chmmtea are either highly watemwluhle or are effectively insoluble. The leae well known chromates were either expensive to synthesize or could not be produced in sufficient purity and/ or yield: “Zinc yellow,” or so-called zinc chromate, having an approximate formula of K20.4Zn0.4Cr08.3H,0, WBB the first wmmeroially successful soluble o h m a t e pigment. This particular salt was h t suggested as a pigment about the time of the fust world war. However, it did not come into general w until the introduction of synthetic phenolic vehiclea in the early 19.30‘8. Combined with these vehicles zinc yellow pmduced an &ective corrcniorwpreventhg primer. It bad achieved some acceptance among paint formulators unW the second world war skyrocketed its consumption in the militmy field. Even with other successful pigments on the market, the search wntinued.for a more nearly ideal soluble chromate pigmeut which would he lm expensive to use and which would liberate more chromate ion than the existingchromate pmducts. National Lead Company research chemists became convinced early in their inveatigations that the m w e r lay in complex salts and they
Runarch Loborotorks, Notional L e d Campany, Brooklyn, N. Y.
synthesized and teated hundreds of double and triple chromate salts. Barium potassium chromate was finally selected BB the most effective because of its relatively low water solubility and the fact that it made available by solution, at a controlled rate, from 20 to 25% more chromate ion per unit weight than the other soluble chromate pigments based on a standard paint formulation. This added chromate concentration greatly enhances the anticorrosive action of paints formulated with bariuni potassium chromate pigment or allows the w of a smaller percentage of active chromate for R given anticorrosive activity in mixed pigment formulas. Althougb barium potassium chromate is a true double salt, apparently crystallized in R regular tetragonal system, it hydrolyzes slowly on exposure to mokture to releaee only potassium chromate. Almost all of the barium chromate is left behind 88 a residual coating with substantial protective properties. Furthermore, the metals potassium and barium do not promote the polymerization of the vehicle oils caklfically BB do most heavy metals. An unexpected advantage was the low tinting strength of the barium potassium salt; this make8 it suitable for inclusion in hoiah mats in which the coloring effect is obtained by other pigments. For some applications such mixed pigment paints can he made to serve both as anticorrosion primers and cow coats. The original batches of barium potsssium chromate were, of course, made at the National Lead Company’s research laboratories in stationary wmbustion trays, placed in laboratory furnaces. However, once the effectivenesa of the pigment had bern
2376
November 1949
. i)
INDUSTRIAL
A’ND
ENGINEERING CHEMISTR?
2377
demonstrated, the technique of economic commercial production came under consideration. Production of the new pigment, now trade-named Pigment E, was undertaken in a pilot plant having a small rotary kiln. After many of the production bugs had been eliminated on this scale, the operation was moved to a semiplant in the company’s Perth Amboy, N. J., plant. At present the production of the semiplant is adequate for the market available to the pigment. The manufacturing technique has been perfected essentially and will be translated directly from the small bcale operation to a full size plant as the market matures. NATIONAL LEAD COMPANY’S SEMIPLANT A T PERTH AMBOY, N. J.
Raw Materials. Pigment E is produced at Perth Amboy by a high temperature reaction bktween potassium dichromate and barium carbonate. The dichromate is obtained as “technical, fine crystalline” from Mutual Chemical Company. Specifications require that i t be better than 99.9% K2Cr20,with less than 0.06% chloride and 0.01 % sulfate impurities. This crystalline material is pulverized in a high speed, hammer-type, Mikro-mill fitted with a 0.0625-inch screen (11). The mill, driven by a 5-hp. motor, has a capacity of 600 pounds per hour. The same grade of dichromate is commercially available.in an already ground state, but early work revealed that finely divided dichromate is slightly hygroscopic, and when stored for even a relatively short time, will form soft aggregates which are not broken up by conventional mixing equipment. The presence of these aggregates in the reaction mixture prevents the required close contact between the reactants and results in an incomplete reaction. To avoid their formation, National Lead grinds the dichromate continuously just before i t is added to the barium carbonate in the blender. The barium carbonate is obtained as a fine powder of which a maximum of 0.18% is retained on a 325-mesh screen. The s u p glier, Barium Reduction Company, labels it “precipitated sulfurfree grade.” A typical analysis is: % 76.9
0.00 0.40
0.03 0.25
Manufacturing Procedure. The crystalline dichromate is delivered in 100-pound bags which are dumped by hand into the Mikro-mill for the initial grinding. The mill discharges into wheeled hopper trucks through a dust-tight cloth collar which is sealed to the truck by a ring-type closure. Since dichromate is slightly toxic (7, 9, IO),operators wear respirators when handling the material and every precaution is taken to prevent the powder from escaping into the air. It was found that the dustrcollecting, air-relief bag furnished with the Mikromill allowed some of the pulverized dichromate to escape, and it was necessary to enclose the entire bag in a dust-tight sheetmetal cylinder exhausted through a bag-type dust collector ( 2 ) . The hopper trucks are wheeled from the mill andstaken by an elevator to the second floor where the blender is located. Here they are weighed on a platform scale, and the net weight is calculated from the tare stenciled on the side of the truck. The trucks are lifted by a power hoist and swung over the opening of the combination ribbon-and-paddle-type blender (8). The trucks are emptied into the blender through a slide-regulated opening in the bottom. Barium carbonate is added to the blender by hand directly from the 100-pound bags in which it is delivered; 1200 pounds of the carbonate and 1720 pounds of the dichromate comprise the charge. Slight variations in the capacities of the hopper trucks are compensated for by appropriately varying the barium carbonate charge. These proportions represent a slight ewess of carbonate; the stoichiometric quantity would b e about 1170 pounds. The excess has been found to minimize the sticking in the kiln, which will be discussed later.
Mikro-mill with Feed Hopper and Enclosed Air-Relief Bag
This batch gives a, theoretical yield of 2640 pounds. Actual yields are over 99% of theoretical. Mixing. The blender is driven by a 20-hp. motor at 47 r.p.m. Blending continues for 1 hour to ensure intimate, uniform mixing of the two reactants. Microscopic examination has proved very effective in determining the completeness of mixing. Although such examinations are not made as a routine procedure, they are used when setting u p mixing schedules and comparing mixer designs. The effectiveness of this mixing operation is of extreme importance to the efficiency of the ultimate reaction. A nonuniform mixture will not react completely. The blender discharges through the center of the bottom of the bin into a 65cubic foot hopper which in turn feeds back into the Mikro-mill. The mix is passed through the mill again, using the same screen as was used for the initial grinding of the dichromate, to break up any remaining aggregates and produce a kiln feed of uniform particle size. No grinding should occur on this pass because overfine dichromate tends to cause sticking in the kiln. I n full scale operations a special type of mixer will be used for this step, instead of the grinding mill, to ensure the physical integrity of the charge during this step. Although the carbonate is heavier than the chromate (specific gravities: BaC03, 4.43; K2Cr2O7,2.73), because of its smaller average size it is carried over into the dust collector of the mill at a greater rate than the chromate., T o preserve the proper reaction proportions, the fines collected from the grinding of one batch are always added to the subsequent batch. Samples of the feed mixture are taken a t ipfrequent intervals and analyzed for chromium trioxide b y dissolving in hydrochloric acid and titrating against 0.1 N sodium thiosufate. The proportions of carbonate and dichromate in the mix can be calculated from these data, and the efficiency of the mixing operations can be confirmed. Since operating practice has been standardized a t
JNDUSTRIAL A N D ENGINEERING CHEMISTRY
VoL 41, No 11
VI
w
VI Y
I
VI VI Y
"
2 01. I-
:
2
November 1949
I
8
INDUSTRIAL A N D ENGINEERING CHEMISTRY
Perth Amboy, not a single mix batch has been rejected a; below standard. Furnacing. The mill again discharges into the hopper trucks which this time take the reaction charge to the kiln feed hopper. The kiln is fed by a 3-inch, water-jacketed, screw conveyer, which is i n turn fed from the hopper by a dry chemical-type disk feeder to ensure close control of feed rate. The semiplant kiln is 26 feet long with an internal diameter of 27 inches. It is lined with 4.5 inches of Jersey common firebrick, has a pitch of 0.5 inch per foot, and rotates a t 1.07 r.p.m. The smaller kiln on which the original work on the process was done is 8 feet long by 18 inches inside diameter, but operates under identical conditions. The kiln is heated by an open propane jet introduced at the d i e charge end and extending about half way up the cylinder. Under normal operating conditions, a t a feed rate of 300 pounds per hour, it burns about 280 cubic feet of 2700 B.t.u. gas per hour. Air, supplied by a second blower, is added to the gas stream in a low pressure induction mixer (I) in a 25:l ratio of air to gas by volume. The mixer automatically mixes air and gas to any desired ratio by a manually regulated opening between a Venturi in the mixer and a gas zero regulator. Once the desired ratio is secured, the ratio is automatically maintained over a wide range of air pressures. The air preesure to the mixer is regulated manually by adjusting a butterfly valve between the air blower and the mixer. An indicating pointer on the butterfly valve allows an operator to duplicate settings which give desired heating rates as previously established. Under normal operating conditions, the opening between the Venturi and gas regulator is adjusted to provide a slightly oxidizing atmosphere in the furnace. The butterfly in the air manifold is adjusted from time to time to increase or decrease the heating rate. Air is added in excess of combustion requirements to ensure an oxidizing atmosphere in the kiln. Reducing conditions tend to produce some chromic oxide, Cr208, which gives a green cast to the finished product and does not form the soluble chromate ions desired in the finished pigment. The reactants are fed to the kiln a t a rate of about 300 pounds per hour and require about an hour to pass through to the discharge end. There is essentially no drying zone since the feed has less than 1% moisture content and the feed end of the kiln maintains a temperature of about 400" C. About 5 feet into the kiln the charge reaches the melting point of potassium dichromate, 398" C., and the mass tends to become plastic and stick to the sides of the kiln. If stoichiometric proportions of the reactants are used, this sticky mass builds up as a clinker on the kiln walls and will completely fill this section of the cylinder unless it is constantly pried off. Once the clinker forms, the kiln must be cooled and the solid mass removed with an air hammer. However, a slight excess, 2 to 4aJ,, of barium carbonate alleviates this condition, although it does not eliminate it entirely. It is believed that the excess carbonate powder coats the individual globules of molten dichromate and prevents their sbicking together to form a plastic mass. Too finely ground dichromate tends to aggravate the sticking, probably because the smaller globules produced on melting present a greater total surface which cannot be coated by the amount of dry carbonate present. Even when excess carbonate is added it is necessary to make some provision for removing cake from the side of the kiln or preventing its formation. The most effective device for this purpose proved to be an arrangement of chains similar to that used in cement kilns (cover photo). One foot inside the entrance of the kiln a circle of five fastening rings are spaced equidistantly around the interior t-
Process Flow Sheet and Plant Layout for Pigment Manufacture
I
,I
16
2319
I
EO
IB
PI
32
36
Figure 1. X-Ray Diffraction Patterns Show Reaction in Kiln Goes to Completion circumference of the kiln. Four identical circles of fasteners are placed a t %foot intervals down the kiln wall extending to a total of 9 feet into the cylinder. Four-foot lengths of 0.375-inch steel chain are festooned diagonally from a fastener in one circle to the fastener to the left of the fastener opposite it in the next circle. As the kiln turns counterclockwise a sort of a screw action is achieved. Although the charge does stick slightly to the chains; the chains are long enough to drag against each other and there is no tendency to ball. The plastic zone in the kiln is about 4 feet long, ending just about a t the point where the chains are discontinued. At this point the charge has a temperature of about 570" C.; beyond that point the charge assumes the form of small, hard, clinkerlike nodules, Reaction. Potassium dichromate dissociates according to the formula : K2Cr207-+-K2Cr04
+ CrOt
The oxide exists only nascently, being absorbed as fast as it is formed by the reaction: BaCOa
+ CrOa
----f
BaCrO4
+ COZ
The formation of the double salt BaKz(CrO& by the reaction:
0
also must occur instantaneously because x-ray diffraction patterns from partially reacted charges indicate the presence of only potassium dichromate, barium carbonate, and the double salt. The normal, single chromates of potassium and barium are completely absent, as is chromium trioxide. X-ray and chemical tests in the National Lead laboratories on materials produced from the regular kiln charge and fired a t 350" C. indicate that the reaction will go to completion a t that temperature. Since none of the compounds present melt a t this temperature, the reaction must be presumed to occur completely in the solid phase. Since the decomposition point of potassium dichromate is usually given as 500' C . , it must be postulated that a slight dissociation in stable equilibrium does occur a t this temperature. The removal of the
2380
NDUSTRIWE A N D E N G I N E E R I N G CHEMISTRY
Vol. 41, No. 11
;tverage particle size between A aiid 10 microiis. As the air sLrc::iin leaves the top of the mill. t'hc ground product is knocked out, in >L 6-foot cyclone separator, arid t>heair is recycled to the intake o l the blower fan. The entire roller mill system is operated at lrrq than atmospheric pressure t o prevent, the escape of dust i i n d fines. To maintain this negat>ivt.pressure, a centrifugal blo\vci. with a capacity of 1100 cubic fret per minute takes off t,he linc after the main blowe~and discharges i o the atmosphere through :i bag-type dust collector ( 3 ) . The niill pressure is controlled by :I butterfly valvr on this take-off line. The ground pigment is put into 400-pound fibw drums diiwtly from the cyclone through :iii air-lock valve. Before packaging, the pigment is t ed for chromium t,rioritlr content, pH! aiid ratr of solul~ionof chromate ion. The chroiniuin analysis is madr b>-titration \$,it 1.1 sodium thiosulfate, arid t,licl results are checked against the theoretical composition of t l i ~ double salt to drtevt the prtwm('e of chromic oxide, which wsults from local reducing c,onditions ill t,he kiln. The prest'iicc. of such free oxide oa,n usually alpo he detect,ed visually hy t i i c . grecri cast which it imparts 1 o t,he product). pII deterniiiiat i o i i s are niitde on R water slui ~i th :I st,andard meter. I.,aboi,:itory irivest,igations show that v e ~ ysmall percent,ages of frcv potassium dichromate prrsmt in thc salt will lower thr pI1 markedly. If thr pH of the product fdls below 8, it, is aocoptc!ti :IS an indication that the lumaor rraction has not gone to complet,ion. The rate of solution in d(~tc?rminedby shaking a 10-grinn sample of the pigment in 100 nil. of water for 1 hour. The smnplc i s filtered and titrated with thiosulfxtt? for chromium ti.iosidc%. An ahnorinally high chroiiistc rontriit in this water extract. is an indication of fret, potassium dichivmate in the pigment. YROPOSlH) I'HOl)IJ(: r1Oh PIANT
Blender Being Charged with Potassium Dichruniate from Hopper 'I r i g clr
products 01 dissociation by icartioii \\ i t h baiium carhoiiai (J ould then force the equilibrium to the light. until the formation of thr double salt is substantially romplrlti. T h r rate of formation at this tempeiature, however, is quite low, and in the commercial operation a temperature of 670" to 700" C. is used to accclcratr the reaction t o attain economiral produrtion. This temperature> i s reached only a t the hottest pal t of the kiln which begins 11 feet in from the entrance, or about 6.5 feet beyond where the chargv ceases to show evidence of a liquid component. The reaction invaiiably goes to completion. X-ray diffraction patterns from the product show no traces of either barium chromate 01 potassium chromate lattices (Figure 1). The excess barium carbonate added to the charge carries through unchanged since the decomposition temperature of this material is 1450' C. Grinding. The finished pigment comes from the kiln i n yellow to greenish-ydlow nodules vai yiiig in size fiom 0.125 to 0.5 inch in diainetei, After they have air cooled, the nodules are shoveled into a bin from which they are fed into an air stream which carries them into a midget roller mill (6). The air stream is provided by a centrifugal blowri which automatically adjusts its output between 3000 and 6000 cubic feet per minute, according to the differential pressure across the mill. The mill is fed through peripheial slots below the grinding sui face of three vertical rolleis and a grinding ring. The air stream cairies the ground material out through a top discharge. TMo successive "whizzers," horizontal fans with inclined blades, knock oversize material back down into the grinding area. By increasing the speed of rotation of these whizzers the size of t,he particles discharged can be rethr product i. hrld to :ti1 duced. In t h r National T,rad pro
The exploration of the maikrl b\- National 1,ead wii (onipleted about thr same time that thr semiplant was brought into qatisfactoi j- productioii and t h r product was announced as coniinercially availablr in limited amount q , Currently, produc>t io1 1
Specific gravity Bulking value, gal., lb. Weight of 1 gallon 60lid. It>-. Oil absorption, g./100 p. Refractive index Barium oxide (calcd. t o BaO!. b' Alkali oxides (calcd. t o KzO), W Chromium oxide (oelcd. t o CrOa), Combined water Total chlorides (as Cl), % Water-soluble chlorldes (a8 C l i , c; Total sulfates (as Sod, % Water-soluble sulfates (as S O & ) ,50 pH (wat,er slurry of pigment) Organic colors a n d lakes Hardness (fused). Mohs' scale ?
1 AJ3LB2 11. Y
ESTIMATEI) E Q L ~ I l ' l I E N T KEQUIRED P O l ~ N L b I ' E R - D , 4 Y PLANT
3.68 0.032U 30.4 11 6
1.9 34 2 21 0 44.6
Sone 0 01s 0.004
0 13 0 0,;
8.0 Yon? 2
FOR
20,000Quanti t5.
Raymond IIIIJ)Mill S o . 51, %OOO-kb. cayaoity, complete with 60-hp., 1800-r.p.m. enclosed motor, base, and magnetic starter Raymond 4237 Hi-Side ring-roller mill, complete II Dracco 3-unit dust collector and accessories. 1200 cu. ft./min.
2 1
3 2
I
>
Conveyer from elevator t o weigh hoppers .J. H. Day blenders, 42 inches X 10 feet, 25-1ip., 800-r.p.ni. motor Sereiv conveyer, 2000-lb.,'hoiir capauicy Storage-kiln feed hoppere, 18,400-lb. capacity Sueci8.l design variable-meed water-cooled scrc\~'Ieerkr
I 2 . >
I 1 1
1
I 1 I 1
November 1949
*
~
INDUSTRIAL AND ENGINEERING CHEMISTRY
from the semiplant is adequate to meet the developing commercial demand, However, on the basis of experience and data obtained from operation of the pilot and semiplants, National Lead has made plans for a full scale production unit for pigment E to be constructed as soon as market demand outstrips the production capacity of the present installation. The plant will have a capacity of 5000 tons per year based on a 300-day operating year but will be arranged so that it can conveniently be operated at half capacity. The installation will be housed in a 30 x 120 foot building, part of which will be divided into two floors. Processing procedure will be the same as that used in the semiplant, but some changes will be made in process equipment and materials handling technique. The unit charge of' 2580 pounds of' crystalline dichromate will be ground, as used, to about 200 mesh in a swing-hammer type mill (6),with 2000 pounds per hour capacity, and conveyed b y bucket elevators to one of two 60-cubic foot weigh hoppers. From the weigh hoppers, the dichromate will be dumped into'one of two 90-cubic foot ribbon-type blenders along with 1800 pounds of precipitated barium carbonate. Charging of the blender is expected to require about 1.5 hours. As in the semiplant, operating procedure will call for 1 hour of mixing. The weigh hoppers will be refilled while the previous batch is mixing. Proper integration ot operations is expected to permit discharge of one of the blenders every 3 hours. The blenders will discharge alternately to another mixer (12) which will break up any lumps or aggregates by dropping the flow of powder onto the center of a rapidly revolving solid disk having several rows of vertical pins around the peripheiy. This particular equipment has been chosen to ensure that no further grinding occurs a t this point. The mixer discharge will be picked up b y a bucket conveyer which will carry it to one of two 335-cubic foot, 18,400-pound capacity, kiln-feed storage hoppers. Each of these hoppers will hold 12-hours full-capacity feed for the kiln, giving a supye capacity of 24 hours of operation.
2381
Test Fences at Sayville, L. I. There has been' some question as to whether this represents a sufficient safety factor, and the fize of these hoppers may be doubled when the plant is actually built. The production kiln will be 50 feet long with 4.5-foot inside diameter and will be lined with 9 inches of refractory brick. It will discharge 835 pounds of pigment per hour when operating a t full production speed of 60 revolutions per hour. Original design called for the kiln to be fed by a water-cooled screw arranged so as to exclude hot exhaust gases which would preheat the charge and might cause it to stick in the screw. However, the engineering design problems inherent in this'sort of feed appear to be excessive and the ultimate installation will probably employ some type of scatter feeder which will throw the mix into the kiln from a distance. The production kiln will be equipped with chain loops in the first third of its length, similar to those which have proved successful in preventing build up of plastic material in the semiplant unit. The kiln will discharge into the pit of a watc~-cooled screw conveyer, this will transfer the pigment nodules to a bucket conveyer which will lift them to a 700-cubic foot, 17-ton capacity, mill-feed storage hopper, This hopper will Eecd M Raymond, Hi-Side roller mill identical to the one in the semiplant (1). As in the smaller installation, flow through the mill will be effected by air stream; the ground pigment will be knocked out in a cyclone, and the remaining dust will go to a three-unit dust collector (3). The cyclone will discharge intermittently through an airtight rotary valve into a 335-cubic foot bagger-feed hopper. The product will be bagged on a high speed, automatic sacking scale equipped with a dust-tight bag holdcr. FIELD PERFORM i N C E
Tnfiloo Feeder, Feed Hopper, and Screw Feed with Water .la+et
A field testing program conducted by National Lead and cooperating paint manufacturers over a period of 8 years has confirmed the special properties expected from paints made with Pigment E. It has been found to be compatible and stable with almost all paint vehicles and pigments now in commercial use. 11 has an exceptionally low reactivity toward the vehicle acids. This lack of effective basicity accounts, at least in part, for the excellent storage stability of proprietary paints formulated with Pigment E. Because of the nature of its manufacture and the high purity of the raw materials, it is a pure chemical compound of definite composition. Consequently, it is substantially free of the soluble chlorides and sulfates often found in other pigments which tend to promote corrosion of the base metal. Pigment E's soft particles (about 2 on Mohs' scale) make it very easy to grind into the vehicle, and its low tinting strength has made it possible
2382
I N D U S T R I A L A N D E N G I N E E R I N G CHEMISTRY
Vol. 41, No. I I
a vehicle composed of a phenolic dispersion resin and a compatible alkyd has given excellent performance. Pigment E represents 65 t o 7570 of the total pigment content. Surface-treated aluminum and magnesium alloys coated with properly formulated Pigment E primers are still in excellent condition after 2 years a t tide range, submerged salt water, and fresh water. The recently introduced wash primers offer additional applications of barium potassium chromate pigments. The “wash” primers are so-called because the film coat, after drying, is extremely thin and furnishes poor protection by itself. These primers, employing vinyl butyral resin, are used t o afford a bonding surface for conventional oleoresinous anticorrosive paints or vinyl copolymer primers which d o not pormally adhere n-ell to metal. In these primers, pigment, E is used in conjunction with dibasic lead phosphite, red lead, or other basic pigments. Reiated t o the wash primer application is the use of Pigment E for added corrosion protection on vinyl strip coatings. It i5 particularly indicated for this application because zinc is a degradation catalyst for polyvinyls. Panel tests of metal protective primer compositions (4) indicate conclusively that the inclusion of even small amounts of Pigment E, of the order of 0.25 pound per gallon of primer, increases the resistance of the paint to corrosive exposure conditions. FUTURE QF CHROMATE PIGMEUTS
Raymond Roller Mill with Double Whiezer Separatols and Feed Hopper Cork base insulation used to reduce vibration
to use it in finish coats in quantities which provide appreciable positive corrosion resistance without seriously affecting the apparent color of the finish. Pigment E has been incorporated in paints applied to metal surfeces and then exposed out of doors for some 7 or 8 years, in atmospheric, tide range, and submerged marine environments. These exposures cover a wide range of vehicles including several varieties of phenolics, alkyds, combinations of alkyd-chlorinated rubber and coumaroneindene resins. These exposures indicated that on ferrous metals the pigment is most outstandingly effective under conditions of atmospheric exposure. However, when applied to alloys of aluminum and magnesium, it was found to give excellent protection in submerged marine and tide range as well as atmospheric exposures. I n all cases, the protection is believed to be effected by the action of chromate ions made available by controlled solution on the metal surface. Several theories have been advanced to explain the mechanism of this action (6). One theory conisiders the inhibition to be due to the formation of a thin, insoluble chromate Rlm on the metal surface. Another theory, called the electron configuration theory, suggests that chromate ions form an adsorbed layer on the surface of the metal. I n either cme, the final result is that the metal surface is less reactive and less susceptible to corrosion. These tests have also shown (4) that paint films containing Pigment E develop hardness but retain high tensile strength, elongation, flexibility, and impact resistance. The films have greater tensile strength and elongation than those containing any other chromate pigment commercially available. These characteristic properties of Pigment E make it especially suitable for use in force-dried or baked coatings, producing films which are exceptionally free from embrittlement. As a result of this testing program, the general area of application of paints containing barium potassium chromate has been determined. One of the most promising applications is in the formulation of primers for light, metal alloys. For these finishes,
The soluble chromate pigments based on the sound technical foundation of chromate ion passivation will undoubtedly continue to find increased appljcation in certain fields in metal priming. Although at the termination of the wartime shipbuilding boom, yearly sales of this type of pigment dropped, both sales and consumption have enjoyed a steady increase in the postwar era. The new barium potassium chromate, while still an infant in the pigment field, is already receiving an encouraging reception from paint formulators and will soon be ready to take its place among the major anticorrosive pigment primers. The low density of these chromate pigments should secure their application on aircraft where the weight of the paint film is a vital consideration. This low density will also ensure their popularity with paint manufacturers who traditionally buy their raw materials by the pound and sell them by thc gallon; savings in shipping weights are also effected by the use of the lighter pigment. The increased use of aluminum and magnesium alloys should also expand the market for the chromate pigments. The protective effect of chromate ions on magnesium and aluminum surfaces is, if anything, more pronounced than on ferrous metals. BIBLIOGRAPHY
Combustion Engineering Co., Chicago, Ill., Catalog 61, pp. 1-8. Ibid., Catalog 55, pp, 1-8.
Dracco Corporation, Cleveland, Ohio, B d l . 304. Eickhoff, A. J., and Kebrioh, L. M., O$iLial Digest Paint & Varnish Prod. C h h s , 1949, 188-98. Evans, U. R., in “Corrosion Handbook,” pp. 9, 39, New York, John Wiley & Son, 1948. Hauok Manufacturing Co., Brooklyn, N. Y., CU~UZOQ 805, 5M 8-42.
Hope, E. W., “Industrial Hygiene and Medicine,” p. 372, New York, William Wood & Co., 1923. Hughsville Machinery &Tool Co., Hughsville, Pa. International Labor Office. Geneva, “Occuoation and Health,” Vol. 1, pp. 437-48, 1930. Xober. G. M., and Hanson, W. D., ”Diseases of Occupational and Vocational Hygiene,” p. 541, Philadelphia, Blakiston Co., 1916. Pulverizer Machinery Co., Summit, N. J., Catalog, pp. 11-21 Satety Car Heating and Lighting Co., Inc., Entoleter Div., New Haven, Conn., Form 4399-4-49. Speller, F. N., “Corrosion, Causes, and Prevention,” pp. 182-6, New York, McGraw-Hill Book Co., 1935. RECEIVED Beptember 5, 1949.
