The Relation of Defects in Enamel Coatings to Hydrogen in Steel*

Abstract

The effects of hydrogen as functions of time, temperature, and impurities in steel are described and experimentally demonstrated. Experimental evidence is presented to show that the blistering and boiling action over carbide areas in steel are due principally to hydrogen that associates with the carbon. Steel enameling stock contains quantities of hydrogea which may effuse during firing to cause or aggravate such phenomena as "boiling," "primary boiling," "reboiling," "blistering," and "bubbling." Low-temperature effusion of hydrogen contributes to other defects, including "fishscaling," "delayed fishscaling," and possibly "pop-offs," "jumping," "shiners," and some caws of "chipping" and, perhaps, "bursting" of enamel on cooking utensils. These defects have seldom been identified with hydrogen evolution. An indirect effect of hydrogen on "copperheads" and "black specks" is also identified.An exhaustive review of both English and German literature on enameling defects is included. Many observations recorded in the literature are shown to agree with the hydrogen theory. Certain types of inclusions in steel are shown to react with occluded hydrogen to form compounds that will not dissociate appreciably at some enamel-firing temperatures, and critical quantities of the hydrogen are therefore prevented from reaching the enamel coating during firing to cause blistering and related defects.

Weather resistance of porcelain enamels exposed for seven years

“…56 Specimens with a thin second coat of enamel on the back were no t affected in this ·way. 5. Where th e initial coverage was complete on all parts of the panel and where no mechanical damage h ad occurred during exposure, protection of th e metal against corrosion was unimpaired on all specimens after 7 yr of \veathering.…”

“…The colored full-mat enamels that shO\yed pronounced fading at the end of the first year did not change much in appearance during the n ext 6 yrs. 5. Fading of the colored enamels of poor acid resistance was almost equally pronounced at all four exposure locations, even benea,th the surface deposit that formed at St. Louis .…”

“…That hydrogen diffusing through steel may literally explode the enamel from th e opposite suIface h ad been demonstrated previou sly by Zapffe and Sims [5]. The fac t that a number of "fish-scale" type fractures wer e noted on the faces of p anels over lo calized corrosion areas while some metal still remained b eneath indicates that th e fractures could not have b een caused by th e forcin g off of th e enamel by corrosion products, nor could they be caused by buckling of the enamel due to the rclease of compressive stra ins r esulting from a small ar ea of the und erlying metal being r emoved by corrosion.…”

Fifteen-year exposure test of porcelain enamels

“…5. A direct relationship was found between acidresistance as measured by the citric acid spot test and weather resistance as measured by changes in gloss and color, except that some of the red enamels of good acid resistance showed excessive fading after 15 yr of exposure at Lakeland 6…”

“…Norton [5] has shown by a tracer technique that the hydrogen generated from such a reaction will permeate the steel structure at room temperature, and numerous investigators, including Zappfe and Sims [6], have demonstrated that whenever hydrogen diffuses through steel, sufficient pressure can be generated to rupture the enamel on the opposite face. Thus, it appears entirely reasonable that the observed spalling behavior could be caused by hydrogen diffusion.…”