P
The Rockwell hardness number was approximately the same on these six steels as was the A.S.T.M. grain sizemeasurement. The surface roughness as measured by the Surfagage and the Profilometer offered no clue as to the difference in behavior in corrosion resistance. Surface studies by x-ray diffraction were also of no help. At this time the supply of good and bad steels from the original 19 shipments was exhausted, and work had to be stopped until addition11 material could be obtained and evaluated. The Armco Steel Co. agreed to cooperate and supplied panels from I9 different heats picked at random. Again striking differences in corrosion resistance were noted after phosphating and painting. Chemical analyses again failed to show any decided differences. The paints used in the test work previously described were oleoresinous primers and gilsonite-type high bake enamels. I n order to determine whether or not the corrosion resistance would follow the same pattern under automotive finishing systems, panels were painted after phosphating in both the body and sheet metal finishing systems used on General Motors cars. Even with these high quality finishes the difference between the good and bad steels was very pronounced. I t was then decided to test the steels without any treatment other than cleaning. Three of the good steels and three of the bad steels were cleaned with solvents until free of water break and then exposed out of doors. The three steels that were the best when phosphated and painted rusted badly in approximately 3 hours, whereas it required 5 days for the bad steels to corrode. Having exhausted the possibilities of determining differences in the steels, it was decided to try treating the bad steels in an effort to make them behave like the good ones. I t was thought that perhaps occluded gases might be playing a part. Panels were heated to 450'F. before phosphating, but this not only failed to improve the bad steels but also made the good steels worse. Vapor degreasing and electrolytic cleaning in an alkaline solution were also tried, but no improvement was noted in corrosion resistance. Various acid dips were tried-sulfuric, hydrochloric, nitric, and phosphoric-with nitric acid the only one to bring about an improvement in the bad steels. Sand blasting was then tried and this treatment made the bad steels equal to the good steels. I t was not known whether this improvement was caused by a roughening of the surface or by the removal of some contaminant from the surface. Panels were then flat polished giving an extremely smooth surface for
phosphating and painting. This made the bad steels equal to the good steels and indicated that removal of something from the surface was causing the improvement. Thinking that it might be an oxidizable material panels were treated with potassium permanganate and chromic acid, but neither had any effect. Since treatment with nitric acid brought about an improvement in the bad steels, it was decided to investigate this aspect. Both good and bad steels were treated with a 3% nitric acid solution, and the residue from this acid treatment was examined spectrographically. The only difference found was that the residue from the bad steels contained lead while in the good steels none could be detected. The presence of lead on the surface of some of the steels is not inconsistent with the differences observed in corrosion resistance. The lead would have a tendency to inhibit the formation of a good adherent phosphate coating and would thus produce bad results under paint.
O n the other hand when exposed without phosphating or painting the lead on the surface would retard corrosion. The only suggestion thus far as to the source of the lead on the surface has come from men in the steel industry. They believe that it might come from lead linings or lead heating coils in the acid pickling baths. This wide variation in corrosion resistance of cold-rolled sheet steel is of importance to people making accelerated tests since if the bad type steel is used in these tests good metal treatments and good paints can be rejected as urnsatisfactory. I n the same manner, articles manufactured from bad steel can have low corrosion resistance and give rise to customer complaints. The work is being continued and the evidence presented here is with the hope that others will become aware of and interested in the problem and perhaps offer suggestions that might be helpful in future study.
RALPH J. WlRSHlNG General Motors Corp., Detroit, Mich.
lacquers for Army Ordnance Materiel
THERE
is a need for fast drying finishing systems for Army ordnance materiel. Ammunition, guns, tanks, wheeled vehicles, and many components lend themselves to production-line methods. This is fortunate because in times of emergency they must be produced in large quantities. I n times of emergency plants are reopened or cqnverted to the production of ordnance materiel. In many cases a fast air drying finishing system is essential because of lack of ovens, lack of power, the size of the plant which prohibits long drying periods, and in other cases the physical shape, size, and/ or the mass of the end item such as tanks. I n addition to the speed of drying there are a number of other requirements s u ~ h as durability, availability, cost, control and ease of application, and methods of evaluating the quality of the finish during procurement. Confronted with this group of requirements it is self evident why Army Ordnance proceeded to investigate the possibilities of nitrocelhlose lacquers. There still exists in the minds of some the impression that lacquers lack durability when compared with alkyd enamels. If one considers the industrial use of alkyd enamels it will be found that most finishes are baked and generally modified with a nitrogenous resin such as urea or melamine formaldehyde. Most industrial metal finishing lacquers are also of the alkyd resin type but utilize nitrocellulose as the hardening agent. The
essential difference then is in the choice of a hardening agent and whether drying will be accomplished by solvent evaporation or baking. There are many relatively new modifiers for nitrocellulose lacquers that alter and in many cases enhance the properties of lacquerspolymeric plasticizers, monomer modified alkyds, nitrogenous resins, and silicones, etc. Competitive procurement by specification requirements presents an additional difficulty. I t is not enough to be able to describe the product by composition and performance, but it is also essential that there be adequate means of exercising control. The analysis for nitrocellulose and alkyd resin in the presence of each other was not possible until recently. Methods were developed for analysis of both nitrocellulose and phthalic anhydride in the presence of each other. With the availability of these i analytical procedures and by carefully describing other compositions that lend themselves to analysis plus performance requirements, it is believed possible to prepare specifications whereby quality can be adequately controlled. Such has been the experience of Army Ordnance since the promulgation of a number of lacquer specifications. High solvent cost and low solids at the spray gun has been a deterrent to the use of lacquer. The multiple coats necessary to obtain the desired and necessary film thickness for good durability meant VOL. 48,
NO. 8
AUGUST 1936
132 1
more labor and cost. This type problem has been practically eliminated by two expedients-proper choice of solvents and hot spray equipment. Good hot spray equipment has been available for about 5 years. This type equipment lowers the viscosity of the lacquer to spraying consistency by the use of heat, thereby saving solvent and permitting high solids to be atomized by the spray gun. One of the greatest causes of failure of any organic coating is the lack of proper preparation of the substrata. Surface preparation was not always given proper attention. It was recognized during World War I1 that proper metal cleaning and treatment contributed greatly to rhe durability of a paint. Additional emphasis on this phase of work since World War I1 has brought gratifying results. The need for proper metal treatment prior to painting was recognized by both ordnance and industry. With the progress made in metal cleaning and treatment, good performance with lacquer coatings is further ensured. Both the inorganic conversion type and the organic type pretreatment coating represented by Specifications MIL-(2-490, Grade 1, and MIL-C-15328 have given excellent results. Five procurement specifications describe the more important nitrocellulose finishes for Army Ordnance. MIL-P11414 describes a nitrocellulose base, red oxide, rust-inhibiting primer and the key to success of any multiple coat lacquer system. MIL-L-11195 describes a hot spray lustreless lacquer with nine colors plus black and white satisfying color requirements of both Army Ordnance and S a v y Bureau of Ordnance for ammunition paints. TT-P-662 supersedes MILS-10181 and describes a nitrocellulose type surfacer, This product dries very rapidly, sands well, and holds out succeeding coats of lacquer to provide a smooth durable finish. MIL-L-12227 describes a hot spray gloss lacquer, not requiring rubbing or polishing. TT-L31 describes sixteen colors plus black and \rhite of gloss lacquer requiring moderate buffing and polishing for maximum depth and gloss. Ammunition, phosphated and coated with but a single coat of hot spray lacquer MIL-L-I 1195, except for moderate chalking, was in excellent condition after 18 months' exposure. This substantiated extensive panel data on both laboratory and commercial batches of lacquer. Grit-blasted bomb bodies coated with MIL-C-15328 followed by a coat each of MIL-P-11414 and MIL-L-11195, except for moderate chalking, were in excellent condition after 4 years of exposure. A number of tanks finished with the same system during the Korean situation are still in excellent condition. In an exposure study of a system comprised of RIIL-P-11414 followed by
1322
MIL-S-10181 and MIL-L-12277, the panels were in excellent condition with only slight chalking and a few scattered pin point blisters. Vehicles finished with this system were satisfactory in appearance without benefit of rubbing or polishing. A panel exposure study of a system comprised of MIL-P-11414 followed by TT-P-662 and TT-L-31 indicated excellent protection at the end of 3 years' exposure with only moderate chalking. Ordnance experience has indicated
that improvement in modifiers, solvents, methods of application, and methods of analysis has made possible controlled procurement of excellent lacquer finishes. The use of proper metal treatment prior to painting is considered essential in obtaining maximum performance from lacquer systems. Atmospheric exposure data on both panels and Army ordnance materiel indicate a highly durable finish.
C. F. PICKETT
Aberdeen Proving Ground, Md.
Melamine Resins in Automotive Lacquers
THE
use of melamine-formaldehyde resins in nitrocellulose lacquers has been the subject of considerable discussion. I t has been speculated that improved gloss retention and higher solids would be imparted. Although some data have been published and many statements have been made there has been little convincing experimental work reported thus far. Automotive production finishes fall into two classes-nitrocellulose lacquers and alkyd/amino baked enamels. Both have certain advantages and disadvantages. The lacquers generally are sprayed at lower solids, have poorer initial gloss (requiring rubbing and polishing), and poorer gloss retention after the first 6 months of exposure. The lacquers are easily patched by spotspraying and rubbing. They may be dried a t room temperature or forcedried a t a moderate temperature for a short time. A schedule of 30 minutes a t 180' F. is typical. The synthetic enamels are sprayed a t higher solids, have rich initial gloss without rubbing and polishing, and show good gloss retention after 6 months' exposure. The synthetic enamels generally cannot be spot-patched. Damaged work is repaired by sanding and spraying a whole panel. A mild orange-peel effect is generally noticeable. The synthetics must be baked at a higher temperature for a longer time. A baking schedule of 45 minutes at 250' F. is typical. T o establish possible advantages imparted to nitrocellulose lacquers by melamine resins a n investigation was undertaken in which the following variables were studied :
1. Total resin (alkyd and melamine) to 100 Darts drv nitrocellulose,. ransing - from 100 to 1500 parts 2. Melamine resin as 0, 20, and 40% .. of the total resin 3. Baking temperaturesof 180°, 210°, and 250" F. 4. Various amino resin types 5. Variation in the alkyd resin
INDUSTRIAL AND ENGINEERING CHEMISTRY
I
_
6 . Catalysis of the melamine resin The alkyd resins used were of two typcs-a nonoxidizing resin, phthalic anhydride 43% and oil acids 3470, and a dehydrated castor oil type with 3970 phthalic anhydride and 40Y0 oil acids. These resins were introduced in the proportions of 90% nonoxidizing and 10% oxidizing type. (In later experiments these resins were introduced as a 50/50 blend.) Butylated melamine resin 248-8 was incorporated as 0, 20, and 40% of the total resin. Twenty-five units of chemical plasticizer, dioctvl phthalate, were included. Two commercial automotive Iacquers and a baked synthetic enamel were used as controls. The baked synthetic enamel was prepared in the laboratory from a soybeantype alkyd resin, 10% melamine resin modified. This control enamel was baked at 250' F. for 45 minutes. All materials tested were uniformly tinted with carbon black to a light gray. Film properties were compared and weathering tests were conducted. From the data collected, the following conclusions were drawn: Melamine resins in nitrocellulose lacquers d o not require baking schedules higher than the 30 minutes at 180" F. normally employed for automotive lacquers. Baking temperatures higher than 200' F. are not desirable because of excessive discoloration of the lacquers. Patching can successfullv be accomplished only when total resin does not exceed 250 per 100 units of nitrocellulose, 207, of the resin being melamine resin. Total solids a t spray weight can be increased up to 50% above those of normal lacquers. Rubbing and polishing ceases to enhance gloss of the melamine resin lacquer films at total resin of 400 to 100 parts nitrocellulose. No resin was found which would give a better result than Cyme1 resin 248-8. The optimum quantity of melamine resin is 20% of The total resin present.
