• Version
• 152 Downloads
• 1.51 MB File Size
• 1 File Count
• October 27, 2016 Create Date
• October 27, 2016 Last Updated

National Advisory Committee for Aeronautics, Report - Relative Importance of Various Sources of Defect Producing Hydrogen Introduced Into Steel During Application of Vitreous Coatings

When porcelain enamels or vitreous-type ceramic coatings
are applied to ferrous metals, there is believed to be an evolu-
tion of hydrogen gas both during and after the firing opera-
tion. At elevated temperatures rapid evolution may result in
blistering while if hydrogen becomes trapped in the steel during
the rapid cooling following the firing operation gas pressures
may be generated at the coating-metal interface and flakes of
the coating literally blown of the metal. This latter type of
defect is known as fishscaling.

Although the behavior of hydrogen in the coating—steel system
has received considerable study, the relative importance of the
different possible sources of the hydrogen causing the defects
has been principally a matter of conjecture. To determine ea:—
perimentally the relative importance of the principal sources,
a procedure was devised in which heavy hydrogen (deuterium)
was substituted in turn for regular hydrogen in each of five
possible hydrogen-producing operations in the‘coating process.
The gas that was evolved when the coated steel specimens fish—
scaled after firing was collected and analyzed with the mass‘
spectrometer. The content, of the deuterium isotope in the total
hydrogen gas evolved was then taken as a measure of the relative
importance of the source under study.

Several investigations, especially over the past decade,
have established that hydrogen is a major cause of coating
defects when porcelain enamels are applied to a steel base
(references 1 to 3). There have, however, been considerable
uncertainty and difference of opinion as to the major source
or sources of the defect-producing hydrogen.

The purpose of the present investigation was to study the
possible sources and to evaluate their relative importance.
With such information available it was believed likely that
methods or procedures could be devised whereby coatings
free of hydrogen defects 3 might consistently be obtained.
It was also believed likely that the results obtained with
porcelain enamel on steel would be applicable when vitreous—
type ceramic coatings are applied to steel and to the so—
called low—strategic alloys. Because of this connection with
ceramic coatings and the growing importance of ceramic
coatings to the aircraft industry, the investigation as herein
described was performed as a part of a broad study on
ceramic coatings being conducted at the National Bureau of
Standards under the sponsorship and with the financial assist
ance of the National Advisory Committee for Aeronautics.