"There is no doubt that the scale itself is in one sense a preventive against rusting; that is, it adheres pretty closely to the skin of the metal and mechanically excludes the air. The scale itself is inmost cases unoxidizable; it consists approximately of magnetic oxide, which is extremely stable. It is true that if part of the surface be covered with scale and the rest be naked, the naked part will rust more rapidly because of the presence of that scale on the neighboring parts, the scale acting 'by difference of potential' to produce corrosion by electrolytic action. But so long as the scale is intact, it acts similarly to paint, excluding the air." The writer adds: "I do not know that I can fully suggest a remedy—the only thing that occurs to me as worth trying is  to modify the sand-blast

so as to  reduce to a minimum  its  roughening