Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
794~
Method of making surface layers with improved corrosion properties
on articles of iron-chromium alloys, and a surface layer made by
the method
-
This invention relates to a method of improving the corrosion propert-
ies of chromium-alloyed steels.
The alloying of, for example, chromium, molybdenum, silicon or aluminium
into steel in order to improve the corrosion properties of the steel
involves substantial manufacturing C08t8- The corrosion properties
of a steel being determined by the condition of the steel surface, it
is of interest to enrich the steel surface in respect of certain alloy-
ing components, which have a positive effect or the corrosion resist-
ance of the steel. In this way, either the amount of alloying compon-
ents in the steel matrix could be decreased in order to achieve a cert-
ain corrosion resistance, or the corrosion resistance for a given
composition in the steel matrix could be increased.
For obtaining a surface layer on a substrate where the structure and
composition in the surface layer differs from the substrate, one can
choose between a number of known methods, for example metallization,
electrolytic precipitat~on or cladding. The principle here is to apply
the desired surface layer externally to the substrate ~ia, for example,
'` . '
1~Z7940
adsorption, condensation or precipitation on the surface of the
substrate from a gaseous or liquid phase.
According to the present invention a surface layer with substantially
improved corrosion properties is obtained thereby that, distinguished
from the methods mentioned above, an oxide is formed on the surface
of the substrate where the oxidized material is comprised in the
substrate.
The present invention, thus, relates to a method of making a surface
layer with impro~ed corrosion properties on articles of iron-chromium
alloys where the article i8 heated in an oxygen containing atmosphere
with vacuum.
The invention is characterized in that the article is heated in an
oxygen containing gaseous atmosphere with a total vacuum of about
10 1 down to 10 8 mm Hg, preferably about 10 5 mm Hg, to a temperature,
at which diffusion of the most reactive slloying component or compon-
ents, primarily chromium, with respect to its tendency of combining
with oxyeen, to the surface becomes perceptible, and at which temper-
ature the remaining properties of the alloy substantially are maint-
ained, and that the described condition for the article is maintained
until a surface layer containing oxidized chromium has grown to a
thickness of preferably 10 9 - 1a 7 m,whereby the resulting surface
layer with respect to structure and composition transforms by degrees
from a condition on the surface to a condition in the matrix of the
metal alloy.
This invention, thus, distinguished from conventional methods where in
all cases a surface layer is applied externally to a su~strate, relates
to a method, at which the surface layer is made by means of alloying
components comprised in the substrate. The surface layer thereby is
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formed by heating the substrate, which is a chromium steel, in a
gaseous atmosphere at vacuum containing a low and well-controlled
oxygen content.
At heating in gaseous atmosphere with vacuum, a preferential enrich-
ment to the surface of the chromium steel takes place of that alloying
component or those alloying components, which have the strongest
tendency of forming compounds with oxygen present on the fiurface of
the chromium steel. The term oxygen present on the steel surface also
includes the oxygen compr-sed in the very thin oxide film, which
usually exists on the chromium steel surface prior to the heating
in the gaseous atmosphere, and the small but very well controlled
amount of oxygen, which exists in the gaseous atmosphere and reaches
to the surface of the article.
The temperature, to which the article is heated, is chosen so that
diffusion of the most reactive alloying component or components to
the surface becomes perceptible. For iron-chromium alloys the diffusion
takes place above a temperature of about 300C. The temperature also
must be chosen so that other properties characteristic of the metal
alloy are not deteriorated. This temperature is for iron-chromium
alloys below about 600 C, because above said temperature carbide is
precipitated in the grain boundaries, and in certain cases pha6e
transformations can occur. The stated lower as well as the stated
higher temperature, of course, are individual for each type of iron-
chromium alloy, but a suitable heating temperature for the majority
of iron-chromium alloys is between 300C and 600C. A preferred temper-
ature is about ~00 C. Surface layers, thus,with improved corrosion
properties have been observed after heating in gaseous atmosphere in
the range 300C-550C, with optimum results about 475C.
~12794~
The amount of oxygen being present on the surface of the
article is to be adjusted both to the heating temperature and
to the desired properties of the surface layer. In many
applications the oxygen amount level should be so low that
substantially only one of the alloying components of the arti-
cle is capable to react with oxygen For effecting such a low
supply of oxygen to the article surface, normally vacuum
conditions are required, which depending on metal alloy and
surface layer can extend from tenths of millimeters Hg to ultra-
high vacuum conditions (~10 8 mm Hg). The heating can incertain cases also be carried out in protective atmosphere, but
then the difficulties of maintaining a sufficiently low and
well-controlled oxygen level are more obvious.
For obtaining a good result, said pressure range implies a
total vacuum of about 10 - 10 mm Hg A preferred pressure
is about 10 mm Hg. According to the invention, the article
is to be treated at said pressure and temperature until a sur-
face layer containing oxidized chromium has grown to a thickness
of about 10 - 10 m and preferably 10 - 10 m, which,
thus, is an extremely thin surface layer. The time of such a
treatment amounts to a number of hours, as is exemplified in
greater detail below.
The time of heat treatment, exclusive of heating and cooling
periods, is to be adjusted, of course, to the amount of oxygen
in the gaseous atmosphere and to the heating temperature. It
was found that this implies heating times in the range one half
to ten hours, with optimum results at about three hours~
The surface layer thus obtained, with a thickness varying be-
tween 10 and 10 7 m, shows a structure and a composition which
by degrees transforms from a condition in the outermost atom
layer of the surface layer
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to a condition characterizing the matrix of the metal alloy.
The improved corrosion properties are attributed to an obser~ed enrich-
ment of alloying comp~nents, sùch as chromium, molybdenum or titanium
in the surface layers thus formed.
As a consequence of the gradual transformation from substrate to sur-
face layer, the surface layer i6 characterized by good adherence to
the substrate. The transformation further takes place without great
accumulations of defects or other material deficiencies. A further
consequence of the fact that the surface layer is applied to the sub-
strate internally and under vacuum conditions, is that the coat is the
~ame all over the article surface, thufi also in inward bulges and other
surface areas where an external application would be rendered difficult
by shading effects.
A surface layer made according to the present-method on articles of
iron-chromium alloys, which were heated in an oxygen containing gaseous
atmosphere under low pressure, thu6, comprises chromium of a higher
concentration than the article in general, which chromium is oxidized
entirely or partially, and which surface layer has a thickness of
about 10-1 _ 10-6 m, preferably 10 9 - 10 m.
Examples:
I
a) A plate of iron-chromium alloy containing 5% chromium was exposed
on a ~urface of 20 mm to tap water for 2 hours. After 2 hours a
corrcsion current of 10 5 A was obtained.
b) A plate similar to that in a) was exposed according to the invention
to a treatment consisting of heating at 475C for 6 hours at the total
pressure 1,8 x 10 5 mm Hg. The resulting surface layer was characterized
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by a gradual transformation from a high chromium content farthest
outwardly in the surface layer to the low chromium content in the
matrix of the alloy. When 20 mm2 of this treated surface were exposed
to tap water, after 2 hours a corrosion current of 10 7 A was obtained.
The one hundred times lower corrosion current was corresponding also
to a hundred times smaller decrease in weight.
Plates of iron-chromium alloys containing 10% chromium were exposed
on surfaces of 20 mm2 to tap water. The sample, which was exposed to
a treatment according to the invention comprising heating at 480 C for
4 hours at the total pressure 2,0 x 10 5 mm ~g, showed after 36 hours
exposure - ~ corrosion current, which was two hundred times lower
than that of untreated samples after the same exposure time.
III
Plates of lron-chromium a1loys containing 17~ chromium were exposed
A on surfaces of 20 mm to a 1~ NaCl solution in the presence of a gap.
The sample~ which was exposed to a treatment according to the invention
comprising heating at 475C for 6 hours at the total pressure 1,8 x
10 5 mm Hg showed after 11 hours exposure one single pitting, while
untreated samples after the same exposure time showed pitting over
the entire gap area.
In the above examples and ther experimental correspondences is shown,
that a preferred treatment consists of exposing the article to an oxygen
containing gaseous atmosphere with a vacuum of about 3 . 10 5 - 10 5
mm Hg at a temperature of about ~00C for a period of about 4-6 hours.
~1Z79~V
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IV
Plates of three different commercially available stainless steels were
exposed on surfaces of 20 mm at room temperature to a 3% NaCl solution
in the presence of a gap. The samples, which were subjected to a treat-
ment according to the invention comprising a heating at 475 C for 1 hour
at the total pressure 1,5 x 10 5 mm Hg showed after 24 hours exposure
corrosion currents, which for all stainless steels were significantly
lower than for untreated stainless steels after the same exposure time.
The differences in corrosion current for treated ~nd, respectively,
untreated samples are apparent from Fig, 1, which is the result of the
exposure during the first fifteen minutes in 3% NaCl solution and
compositions according to the below Table 1 of the stainless steels
included in the comparison. It should be added that the corrosion curr-
ents during extended exposure tests continue to decrease with the
time, but that the mutual order between different treated and untreated
stainless steels is the same as after 15 minutes exposure~
The surface6 of iron-chromium alloys prior to heating in easeous atmos-
phere can be subjected to some kind of treatment, such as mechanic
polishin~, bright annealing, pickling, electrolyte polishing or etching,
or alternatively they may not be treated. In all cases, however, a
very increased corrosion resistance of a material treated accordine
to the inYention is obtained.
~elow examples are gi~en which refer to bright annealed and, respect-
ively, pickled ~aterial, while the above examples referred to mechanic-
ally ground surfaces.
Brigh~ annealed plates of stainless steel SIS 2343 (as regards composit-
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ion see designation B in Table 1) were exposed on surfaces of 20 mm
at room temperature to 3% neutral NaCl solution in the presence of a
gap and during a simultaneous potentio-dynamic load, which in fiteps
was increased by 50 mV per minute. The potential, at which local attacks
were initiated adjacent the gap,i6 hereinafter called crevice corrosion
potential. The samples, which were exposed to a treatment according
to the invention comprising heating at 475 C for 2 hours at a total
pressure ~f 5 x 10 5 mm Hg showed a crevice corrosion potential, which
on the average was 550 mV hiBher than for corresponding untreated bright
annealed steel samples.
VI
Pickled ~lates of stainless steel SIS 2333 (as regards composition,
see designation D in Table I) were subjected to a similar corrosion
test as de~cribed in Example V. The fiarnples, which were subjected to
a treatment according to the invention comprising heating at 475 C
for 2 hour6 at the total pressure of 7 x 10 ~ mm Hg, showed a crevice
corrosion potential, which on the average was 130 mm hi~her than for
corresponding untreate~ pickled teel samples.
Tabele I. Chemical analysis in % by wei~ht
Design- Material C Si Mr. CrNi r~0 Other
ation
A Cr18Mo2Ti 0,030 0~34 o~48 18.4 0.27 2.27 0.60 Ti
B SIS 2343 o.o46 o.38 1 r30 17,7 11.0 2.74
C Cr18Ni25~oCu 0.020 o-s6 1.82 19.6 24.5 4.4 1.43 cu
D SI5 2333 0.040 0~47 1.67 18.8 9.1 o ~
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_9_
The present invention can be applied to alloys other than pure iron and
chromium alloys. As iron-chromium alloys according to the present
invention are understood alloys, which in addition to the basic elements
iron and chromium contain one or more of the basic elements nickel,
aluminium, silicon, titanium, manganese, copper or molybdenum.
