A study of the weather resistance of porcelain-enamel ed architect ural panels was begun at the National Bureau of Standards in 1939. The s tudy in vol yes 864 I-foo t-square panels and a like num ber of 4-b y 6-illch laboratory specimens. A previou s report (H)42) gave t he res ults after 1 year of expos ure. The prese nt paper describes t he condition of the panels after 7 years of weathering at Washin gton, D . C., St. Louis, Mo. , Lakeland, Fla., a nd Atlantic City, N. J.The res ults of t he 7-year inspection showed a good correlation between the ac id resistance of enam els and their resistance to weatherin g. \Vh ere ini t ial coverage was complete and no m echani cal damage had occulTed, protection of th e m e tal by the ell a rn el for all specim ens \\:1S unimpaired aft e r 7 years.Fading of colored en amels occ urred only on t hose pa nels with \'er y poo r acid res i~ta'lce .

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A new type of ceramic coating for the protection of low-carbon steel in high-temperat ure service was d eveloped during the war at the National Bureau of Standards and was used by the Army and Navy on the exhau st system s of certain a ircraft and other vehicles. The outstanding features of these coatings are (a) high resistance to chipping unde r repeated thermal hock, (b) protection of the metal against ox idation during prolonged expo. ure at temperat ures up to about 1,250° F , (c) freedom from the crac kin g and blistering produced in conventional porcelain enamels under comparable conditions of high temperature and severe Lhermal gradients, and (d) a mat surface wh ich does not show high lights and , t herefore, decreases t he visibility .

SUMMARYPorcelain-enamel ground coats were prepared and applied under conditions that gave various degrees of adherence between enmel and a low-carbon steel (enameling iron). The variations in adherence were producedby (a) varying the amount of cobalt-oxide addition in the frit, (b) varying the type of metallic-oxide addition in the frit, keeping the amount constant at 0.8 weight percent, (c) varying the surface treatment of the metal before application of the enamel, by pickling, sandblasting, and polishing, and (d) varying the time of firing of the enamel containing 0.8 percent of cobalt oxide.Specimens of each enamel were given the standard adherence test of the Porcelain Enamel Institute.' MetaUographic sections were made, on * which the roughness of interface was evaluated by counting the number of anchor points (undercuts) per centimeter of specimen length and also by measuring the length of the interface aud expressing results as the . ratio of this length to the length of a straight line parallel to the over-all direction of the interface.The following conclusions were drawn from the data:(1) A positive correlation was found between the adherence of a porcelain-enamel ground coat end the roughness of the interface.(2) In general, adherence correlated better with anchor points per centimeter than with the increase in interracial uea (interface ratio).(3) The method of metal preparation had a marked effect on the relation between roughness of interface and adherence of porcelainensmel ground coats to enameling iron. In general, better adherence was associated with enamels applied to pickled iron than to sandblasted iron for the sane degree of roughness of interface.. (4) Most of the ro~hness that was associated with good adherence between a porcelain-enamel ground coat and iron developed during the . firing process.(5) Roughness of interface is a necessary, but not a condition for the development of good adherence between a enamel ground coat and iron.NACA TN 2934 sufficient, porcelain-(6) One or more factors other than roughness of interface also influence the adherence between a porcelain-ensmel ground coat and iron.