PROTECTING FERRITIC STAINLESS STEEL FROM CREVICE AND PITTING CORROSION

PROTECTION DE L'ACIER INOXYDABLE FERRITIQUE CONTRE LE CRIQUAGE ET LE PIQUAGE PAR CORROSION

English Abstract

ABSTRACT OF THE DISCLOSURE
A process for protecting ferritic stainless steel
against crevice and pitting corrosion in an environment corrosive
thereto. The ferratic stainless steel is protected against
crevice and pitting corrosion by covering a portion thereof with
another stainless steel which is less noble than the ferritic
steel in the referred to corrosive environment.

Claims

The embodiments of the invention in which an
exclusive property or privilege is claimed are defined as
follows:
1. A process for producing a vessel of ferritic stain-
less steel which is protected against crevice and pitting
corrosion in an environment corrosive thereto, which process
comprises the steps of: providing a ferritic stainless steel
substrate and covering separate portions of one surface of
said substrate with a dissimilar stainless steel, said dis-
similar stainless steel being less noble than said ferritic
stainless steel in said corrosive environment, said dissimilar
stainless steel being in electrical contact with said ferritic
stainless steel and forming said ferritic stainless steel into
a vessel suitable for containing a liquid medium, said dissimilar
stainless steel being on the inner surface of the vessel.
2. A process according to claim 1, wherein said ferritic
stainless steel is an A.I.S.I. Type 439 stainless steel.
3. A process according to claim 2, wherein said
dissimilar stainless steel is an A.I.S.I. Type 409 stainless
steel.
4. A vessel shaped article comprising a ferritic stain-
less steel substrate and a dissimilar stainless steel attached
thereto so as to the in electrical contact therewith, said
dissimilar stainless steel covering separate portions of the
inner surface of said substrate, said dissimilar stainless
steel improving the crevice and pitting corrosion resistance of
said ferritic stainless steel in an environment corrosive thereto,
said dissimilar stainless steel being less noble than said fer-
ritic stainless steel in said corrosive environment.

5. An article according to claim 4, wherein said ferritic
stainless steel is an A.I.S.I. Type 439 stainless steel.
6. An article according to claim 4, wherein dissimilar
stainless steel is an A.I.S.I. Type 409 stainless steel.

Description

1027
1 PROTECTING ~ERRITIC STAINLESS STEEL FROM
. _ .
CREVICE AND PITTING CORROSION
~BSTRACT OF T~E DISCLOSURE
A process for protecting ferritic stainless steel
against crevice and pitting corrosion in an environment corrosive
thereto. The ferritic stainless steel is ~protected against
crevice and pitting corrosion by covering a portion thereof with
another stainless steel which is less noble than the ferritic
steel in the referred to ~orrosive environment.
1~
The present invention relates to ~ process or
protecting ferritic stainless ste~ls against crevice and pitting
~orrosion.
Although ferritic stainless steels are excellent
~aterial considerations for water containing vessels, chloride
and ~ineral contamination in some waters is sufficiently high to
promDte crevice and pitting corrosion. This is especially true
in 301ar ~bsorber panels which are characterîzed by severe
~reviees.
Through the present invention, there is now provided
an i~proved Deans for protecting ferritic stainless steel from
crevice and pitting corrosion. The ferritic stainless steel is
protected by covering a portion thereof with another stainless
steel ~hich is less noble than the ferritic steel ~n the corrosive
ænvironment.
Design engineers have ~tte~pted to protect ferritic
~tainless ~teels from ~revi~e and pitting corrosion by covering

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1 portions thereof wi~h a dissimilar class of metal. United
States Patent No. 3,830t7~0 discloses a process wherein aluminum
i5 used to protect stainless steel. The aluminum forms the
cover of a sacrificial anode. The core of the anode is stainless
S steel. ~s the stainless steel core is of the same ~he~istry as
the stainless ~teel to which lt is attached, the patent does
not disclose a process wherein ferritic stainless steel is
protected by a dissimilar stainless steel.
Many other references deal with galvanic reactions.
These references include United States Patent Nos. 1,613,461,
2,303,778, 2,619,455, 3,084,742, 3,378,359, 3,616,416 and
3,830,634. None of them provide a process wherein a dissimilar
stainless steel is used to protect a ferritic stainless st~el.
By utilizing a dissi~ilar stainless steel, the present invention
prolongs the life of the sacrificial metal and reduces the
amount of contaminating corrosion products.
It is accordingly an object of the present invention
to provide a process for protecting ferritic stainless steel
against crevice and pitting corrosion.
The present invention sets forth a process for protecting
erritic stainless steel against crevice and pitting corrosion
in an environment corrosive thereto. It comprises the steps of
providing ~ ferritic stainless steel substrate and covering
6eparate portions of one surface ~f ~he substrate with a dissimilar
stainless steel which is less noble than the ferritic stainless
~teel in the referred to corrosive environment. The corrosive
environment is generally a ~hloride-bearing environment~ It
~ould ~lso be ~n environment ~hich is ~ulfide-bearing or caustic.
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1 The ~hoice ~f the dissi~ilar ~tainless ~teel is dependent upon a
determination of the environment to which the ferritic stainless
steel ~s ~o be subjected. The dissimilar stainless steel can be
attached o the ferritic stainless ~teel in a variety of ways
which`include welding, pressure bonding, Yiveting ~nd spraying.
~he steels must be ~$~eh~ so as to provide $ntimate electrical
contact.
The article of the present invention is characterized
by a ferritic stainless steel substrate and dissimilar stainless
steel attached thereto. The dissimilar stainless steel covers
separate por~ions of one surface of the subs~rate, and is as
described hereinabove. As noted in the examples appearing
hereinbelow, A.I.S.I. Type 409 stainless steel can be the
dissimilar steel used in conjunction with A.I.5.I. ~ype 439
stainless steel. The article ~an be formed and welded into
various vessels such as solar absorber panels and hot water
tanks. Alternatively, the dissimilar stainless steel can be
~ttached to the ~erritic stainless steel during or subsequent to
fabrication. The dissimilar stainless steel constitutes a
portion of the inner surface of the finished vessel~
The following examples are illustrative of several
aspects of the invention.
To demonstrate the present ~nvention, a co~posite was
~ade by spot welding an A.l.S.I. Type 409 specimen between two
~5 A.I.S.I. Type 439 specimens. Type 409 ~tainless ~teel is less
noble ln chloride-bearing environments than is Type 439 stainless
steel~ An additional co~posite was ~ade using ~luminum in place
of the 409 stainless ~teel. As discussed hereinabsve, aluminum

1 has been used to protect ferritic stainless steels from crevice
and pitting corrosion.
The co~posites were placed in hot water at a temperature
of 180F. Sodium chloride was added to the water to provide a
~hloride ion concentration of 1000 ppm. After 8 ~eeks the 439
having the 409 welded thereto was basically in as good a condition
~s was the 439 having the aluminum welded thereto. The weight
loss for the 439-409-43~ composite was, however, significantly
less than that for the 439-Al-439 ~omposite. The percent loss
was 0.38~ for the 409 composite and 1.01~ for the composite
having aluminum. Significantly, the present invention provides
an advantageous means for protecting ferritic stainless steel
from crevice and pitting corrosion.
It will be ~pparent to those skilled in the art that
the novel principles of the invention disclosed herein in
connection with specific examples thereof will suggest various
other modifications and applications of the same. It is accordingly
desired that in construing the breadth of the appended claims
they shall not be limited to the specific examples of the
invention described herein.
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