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National Advisory Committee for Aeronautics, Technical Notes - High Temperature Attack of Various Compound on Four Heat Resisting Alloys

In the preparation of suitable protective ceramic coatings for heat-
resisting alloys, indications were that certain of the comon coating
ingredients were reacting with the alloys and thereby limiting the life
of the coated specimens at the temperatures of operation. This effect
was acute in the case of Hastelloy B alloy. In order to obtain a pre-
liminary indication of the identity of the ingredients that were causing
the difficulty, a total of 61 conmounds, all of which contained elements
that have been or might logically be used in coatings, were tested by
placing a small pulverized sample of each compound on the cleaned surface
of an alloy specimen and heating in air for 17 hours at 15000 F. Visual
examinations were then made to determine the degree of attack, if any
The results indicate: (1) That Hastelloy B is much more susceptible to
heavy attack by corrosive coating ingredients when heated in air than the
other three alloys studied (S-816, 8-590, and Haynes Stellite No. 21),

(2) that of the comonents that reacted with the alloys, the alkalies,
lead compounds, and some of the alkaline earths gave the most pronounced
attack, (3) that the attack of the alkalies on Hastelloy B does not occur
in a carbon-dioxide or a. helium atmosphere, and (1+) that it should be
possible to prepare coating compositions which will permit the ceramic
coating of high-molybdenum alloys to prolong their life under operating
conditions which now produce early deterioration.

It is logical that some or all-alloys, when protected with heat-
resisting, tightly adherent, ceramic coatings might be operated safely
at higher gas temperatures than can be endured for a sufficiently long
period by the uncoated alloys. The potential usefulness of such a '
coating is, of course, greater for alloys which are readily attacked by
atmospheres encountered in service.

One of the problems in the development of stable coatings for
continued use at high temperature is the preparation of compositions
that do not hamfulJy react with the alloys The early attempts to apply
ceramic coatings to Hastelloy B alloy in this study made it obvious that
such reactf ii was occurring. The elimination of this reaction seemed.
possible, however, by identifying the reactive ingredients and excluding
them from the coating.