Cause of Pinholes and Some Related Defects in Enamel Coatings on Cast Iron*

Abstract

The confusion in identifying hydrogen as the predominating cause of certain defects in enamel on cast iron has been due largely to the close association of carbon and hydrogen in cast iron and steel. The principal relation of carbide and graphite to enameling defects is the release of hydrogen from the carbon during enamel firing. The much-discussed "chill layer" therefore is important chiefly because this layer often contains hydrogen that is bound to the carbon in the cementite. Experiments show that when hydrogen is absent, regardless of the depth or nature of the surface chill, no pinholing or blistering results during firing at 725°C.Sources of the hydrogen that causes the defacement are found chiefly in melting and in casting. The low oxygen pressure of molten cast iron favors hydrogen absorption. Moisture in the atmosphere, in the charge, or chemically combined in the rust on scrap provides thegreatest quantities of thegas,>nd moisture and organic materials in the mold are also prolific sources of hydrogen for absorption by the iron. At ordinary temperatures, rusting is often harmful.Flushing the melt with a dry, hydrogen-free gas, such as nitrogen, removes the dissolved hydrogen, and defects during subsequent enameling will not occur unless hydrogen is obtained later from other sources.Chipping phenomena probably are caused chiefly by hydrogen effusion, just as are analogous defects in sheet-steel enameling.

Thermodynamics and phase stability in the Si–O system

“…XRD Mixture, not crystalline, maybe stable at higher temperatures 1942 [193] * Electric furnace, acid slag Chemical analysis, XRD Compound 1942 [194] * SiO dissolved in steel Slag reactions Compound 1947 [195] * SiO in production of Fe x Si y in electric furnace…”

Hydrogen Embrittlement Understood

“…High hydrogen pressure bubble or void; [20][21][22] Hydrogen-induced reduction in surface energy; [23,24] Hydrogen-enhanced dislocation ejection from the surface or near surface region; [17] Hydrogen-induced reduction in cohesive strength; [6,18,[25][26][27][28] Hydrogen-enhanced localized plasticity; [7,9,[29][30][31] Hydrogen-and deformation-assisted vacancy production; [32][33][34] Hydrogen-triggered ductile to brittle transition; [35,36] Hydride formation and cleavage; [2,3] Hydrogen-and strain-induced phase transformations; [37,38] and Reactants and hydrogen. [39] Of these possibilities, the operative ones depend on the material, the hydrogen charging conditions, and the loading.…”

L'hydrogène dans les matériaux métalliques en relation avec les interactions plasticité-environnement

“….) qui jouent le rôle de pièges [55,56]. Ces phénomènes sont observés pour des conditions très sévères d'exposition des aciers à l'hydrogène, notamment dans l'industrie pétrolière (milieu H 2 S) [57].…”