Note: Descriptions are shown in the official language in which they were submitted.
METHOD FOR PASSIVATING METAL SURFACES
FIELD OF THE INVENTION
The present invention is directed to a novel method to passivate metals, more
specifically it is
directed to a method using a modified Caro's acid to passivate metals.
BACKGROUND OF THE INVENTION
Metal passivation, in the fields of chemistry and engineering, refers to the
generation of a coating
on a material to render the latter less prone to being affected or corroded by
its surrounding environment.
This coating renders the surface of the material passive.
The coating which renders the material passive is formed by a chemical
reaction with the base
material, or in some cases, it is formed by exposing the material to air so as
to oxidize it. Generally, the
coating involves a metal oxide which is quite unreactive and hence ideal for
exposure to various solutions
and environments. This layer of metal oxide provides protection against
corrosion, which would decrease
the material's lifespan.
Metal passivation is a post-fabrication method of maximizing the inherent
corrosion resistance
of a stainless alloy from which the workpiece was produced. It is not a scale
removal treatment, nor is it
like a coat of paint. Passivation provides valuable corrosion resistance of
parts and components machined
from stainless steels. Properly conducted passivation can prevent premature
failure of metals, but when
not properly conducted, it may actually provide weak points for corrosion to
begin.
Passivation can be best understood as a deliberate-controlled corrosion. Once
in a passivation bath,
the acid dissolves, or corrodes, free iron (or other alloy metals) at the
surface of the metal. This is done in
a controlled and uniform manner so long as the bath parameters, such as
temperature, acid concentration
and exposure time, are followed. When the reaction is not properly overseen, a
runaway reaction of
corrosion can occur and is referred in the industry as a "flash attack." When
a metal undergoes a flash
attack, it can develop a dark and damaged or etched surface.
Proper passivation requires at least two steps: the cleaning of the metal
followed by exposure of
the metal to a passivation bath.
1
Date Regue/Date Received 2022-08-12
Cleaning metals prior to exposing them to a passivation bath is a critical
step, as various
contaminants, however small they may be, can find their way onto a metallic
surface and generate weak
points on the metallic surface where corrosion may take hold. Contaminants
such as grease, coolant or
other shop debris must be thoroughly cleaned from the surface to obtain the
best possible corrosion
resistance. It is important when performing passivation of metals that a
number of steps be taken
beforehand. With respect to various metals which have been machined, machining
chips must be
removed, and this can be simply by wiping off the metal piece. A subsequent
step can include the use of
a commercial degreaser or cleaner to remove various oils or fluids used in the
machining step.
Should the oils not be removed or not be properly removed, one can expect the
formation of gas
bubbles on the surface of the metal being passivated. The presence of gas
bubbles will impede the
passivation and may create weak spots on the passivated metal piece. In some
cases, an accumulation of
chlorides can lead to a "flash attack" which leaves a heavily etched or
darkened surface, in essence,
damage to the surface which was destined to be protected through passivation.
It becomes evident that proper steps need to be taken when understanding that
these passivated
metals are used in many critical industries including, but not limited to,
aerospace, medical and dental,
and various other industries.
Passivating Baths
After thorough cleaning, the stainless-steel part is ready for immersion in a
passivation acid bath.
A number of conventional approaches are currently widespread in the field:
nitric acid passivation, nitric
acid with sodium dichromate passivation, and citric acid passivation.
The preferred passivation approach typically depends on the type of stainless
steel and the
acceptance criteria. Today, both nitric acid and citric acid passivation are
widely used and relied upon in
many industries. Both passivation approaches have shown effectiveness in
passivating numerous types of
stainless steel and are extensively described in the industry standards ASTM
A967 and AMS 2700. It is
worth noting that passivation of titanium is governed by ASTM F86, which only
specifies passivation by
exposure to nitric acid.
Stainless steels which can be passivated using citric acid at temperatures
above 120 F, include:
Austenitic: Martensitic-PH; Ferritic (type 430); Martensitic; and Austenitic-
FM. Stainless steels which can
2
Date Regue/Date Received 2022-08-12
be passivated using citric acid at temperatures below 120 F, include: Ferritic-
FM (Types 430F & 430FR);
Ferritic-FM (Chrome Core 18-FM); Ferritic-FM (Type 409Cb-FM); and Martensitic-
FM (Type 416).
Stainless steels which can be passivated using nitric acid (typically at 20
vol.%) at temperatures
above 120 F, include Austenitic stainless steel. Stainless steels which can be
passivated using nitric acid
(typically at 20 vol.%) at temperatures below 120 F but in the presence of
sodium dichromate (Na2Cr207),
include: Martensitic-PH; Ferritic (type 430); Martensitic; Austenitic-FM;
Ferritic-FM (Types 430F &
430FR); Ferritic-FM (Type 409Cb-FM); and Martensitic-FM (Type 416).
Nitric Acid Passivation
Nitric acid is a mineral acid which is a hazardous chemical in that it emits
toxic and corrosive fumes.
It requires special attention to ventilation and safe chemical handling. It is
highly reactive to skin and can
rapidly cause severe chemical burns. Nitric acid emissions in the atmosphere
have been associated with
acid rain and smog and can even impact the ozone layer. These and other
factors explain the high degree
of oversight with respect to nitric acid and its handling and use, which
translate into rigorous requirements
for safety in handling and environmental protection.
Stainless steels which may be more prone to etching (flash attack) during the
passivation in nitric
acid can benefit from the addition of sodium dichromate passivating
composition. Other options to reduce
the risk of flash attacks during nitric acid passivation include the use of
higher concentrations of nitric acid
as well as higher bath temperatures. While these options to reduce flash
attacks may work, they also increase
the risk to personnel safety. It is worth noting that sodium dichromate is a
hexavalent chromium compound,
which is a known carcinogenic substance.
Citric Acid Passivation
In comparison to nitric acid passivation, citric acid passivation offers a
generally safer and more
environmentally friendly approach to passivation.
Citric acid is an organic acid which is safer for operators than nitric acid
as it is low fuming and
much less toxic than its nitric acid counterpart. Citric passivation is widely
used as it meets current industry
standards and is applicable to most types of stainless steel.
In the past, some manufacturers regarded citric acid passivation negatively
due to the possible
mould growth in the citric acid baths. Nowadays, citric acid passivation has
overcome this perception, since
3
Date Regue/Date Received 2022-08-12
some formulations now contain biocides aimed at preventing organic growth and
mould. In fact, citric acid
passivation has become increasingly popular with manufacturers who want to
want to steer clear of the
nitric acid compositions and especially those containing sodium dichromate.
These compositions can be
very difficult to dispose of and are subject to more regulations than citric
acid. Citric acid is considered
environmentally friendly, which explains its widespread acceptance.
Nonetheless, citric acid passivation requires close monitoring of immersion
time, bath
temperature and concentration in order to avoid "flash attack" which would
otherwise mar or damage the
metal being passivated.
In fact, laboratory tests have shown that passivation with citric acid was
more likely to cause
"flash attack" than passivation with nitric acid. Some of the reasons for the
occurrence of flash attacks
can be found in excessive bath temperature, excessive immersion time and
contaminants present in the
baths. Citric acid compositions used in metal passivation contain chemicals
such as, but not limited to,
corrosion inhibitors and other additives (such as wetting agents) that are
said to minimize the occurrence
of flash attacks.
It has been noted that another advantage of citric acid passivation is that it
involves, in many cases,
shorter passivation times versus nitric acid passivation.
When passivating metals using citric acid, it is important to monitor the
temperature, the time of
immersion and the concentration of the acid so as to avoid any possible
runaway corrosion referred to as
a flash attack. Some advantages of using citric acid passivation rather than
nitric acid passivation include
the following:
- no emission of toxic and corrosive gases during the process;
- lower handling requirements in terms of safety equipment;
- lower concentration of acid is required;
- lower overall cost to use;
- process can be carried out a room temperature in the case of some
metals and without the
need for ventilation;
- uses environmentally friendly chemicals, especially important when it
comes time to dispose
of spent acid; and
- the process does not lead to corrosion of the equipment employed.
4
Date Regue/Date Received 2022-08-12
In terms of overall process stability, citric acid passivation is less prone
to time and temperature
variation than nitric acid passivation. Furthermore, citric acid passivation
does not emit any hazardous
vapors.
In light of the state-of-the art, there exists a need for a new composition
which can be used for the
passivation of stainless steel which shows advantages not shared with
conventional composition and
methods associated therewith. The inventors have surprisingly and unexpectedly
discovered a new acid
composition and method to passivate metals using a composition which overcome
some of the drawbacks
of each one of the known and conventionally used passivation methods.
SUMMARY OF THE INVENTION
According to an aspect of the present invention, there is provided a method
for passivating a
metallic surface, said method comprising:
- providing said metallic surface;
- exposing said metallic surface to a modified Caro's acid composition
selected from the group
consisting of: composition A; composition B and Composition C;
wherein said composition A comprises:
- sulfuric acid in an amount ranging from 20 to 70 wt% of the total weight of
the
composition;
- a compound comprising an amine moiety and a sulfonic acid moiety selected
from the group consisting of: taurine; taurine derivatives; and taurine-
related
compounds; and
- a peroxide;
wherein said composition B comprises:
- an alkylsulfonic acid; and
- a peroxide; wherein the acid is present in an amount ranging from 40 to 80
wt%
of the total weight of the composition and where the peroxide is present in an
amount
ranging from 10 to 40 wt% of the total weight of the composition;
wherein said composition C comprises:
- sulfuric acid;
- a compound comprising an amine moiety;
- a compound comprising a sulfonic acid moiety; and
- a peroxide;
Date Regue/Date Received 2022-08-12
for a period of time sufficient to coat said metallic surface with a metal
oxide film created by the
exposure of said metallic surface to said modified Caro's acid composition.
According to a preferred embodiment of the present invention, said sulfuric
acid, said compound
comprising an amine moiety and a sulfonic acid moiety and said peroxide are
present in a molar ratio of no
less than 1:1:1. Preferably, said sulfuric acid, said compound comprising an
amine moiety and a sulfonic
acid moiety and said peroxide are present in a molar ratio of no more than
15:1:1. According to another
preferred embodiment of the present invention, said sulfuric acid and said
compound comprising an amine
moiety and a sulfonic acid moiety are present in a molar ratio of no less than
3:1.
Preferably, said compound comprising an amine moiety and a sulfonic acid
moiety is selected from
the group consisting of: taurine; taurine derivatives; and taurine-related
compounds. According to a
preferred embodiment of the present invention, said taurine derivative or
taurine-related compound is
selected from the group consisting of: taurolidine; taurocholic acid;
tauroselcholic acid; tauromustine; 5-
taurinomethyluridine and 5-taurinomethy1-2-thiouridine; homotaurine
(tramiprosate); acamprosate; and
taurates; as well as aminoalkylsulfonic acids where the alkyl is selected from
the group consisting of CI-Cs
linear alkyl and C1-05 branched alkyl. Preferably, said linear
alkylaminosulfonic acid is selected form the
group consisting of: methyl; ethyl (taurine); propyl; and butyl. Also
preferably, said branched
aminoalkylsulfonic acid is selected from the group consisting of: isopropyl;
isobutyl; and isopentyl. More
preferably, said compound comprising an amine moiety and a sulfonic acid
moiety is taurine. According
to a preferred embodiment of the present invention, said alkylsulfonic acid is
selected from the group
consisting of: methanesulfonic acid; ethanesulfonic acid; propanesulfonic
acid; butanesulfonic acid;
pentanesulfonic acid; hexanesulfonic acid; and combinations thereof.
According to a preferred embodiment of the present invention, said sulfuric
acid and compound
comprising an amine moiety and a sulfonic acid moiety are present in a molar
ratio of no less than 3:1.
According to another aspect of the present invention, there is provided a
method for passivating a
metallic surface, said method comprising:
- providing said metallic surface;
- exposing said metallic surface to a composition comprising:
- an alkylsulfonic acid; and
- a peroxide;
wherein the acid is present in an amount ranging from 40 to 80 wt% of the
total weight of the composition
and where the peroxide is present in an amount ranging from 10 to 40 wt% of
the total weight of the
6
Date Regue/Date Received 2022-08-12
composition.
Preferably, the method uses an aqueous acidic composition comprising:
- an alkylsulfonic acid; and
- a peroxide; wherein said alkylsulfonic acid; and said peroxide are present
in a molar ratio of no
less than 1:1. Preferably, said alkylsulfonic acid is selected from the group
consisting of: methanesulfonic
acid; ethanesulfonic acid; propanesulfonic acid; butanesulfonic acid;
pentanesulfonic acid; hexanesulfonic
acid; and combinations thereof. More preferably, said alkylsulfonic acid is
methanesulfonic acid.
According to a preferred embodiment of the present invention, the method
further comprises a
compound comprising an amine moiety. Preferably, the compound comprising an
amine moiety is a
primary amine. More preferably, the compound comprising an amine moiety is an
alkanolamine. More
preferably, said alkanolamine is selected from the group consisting of:
monoethanolamine; diethanolamine;
triethanolamine; and combinations thereof.
According to a preferred embodiment of the present invention, the compound
comprising an amine
moiety is a tertiary amine. More preferably, said alkanolamine is
triethanolamine.
According to an aspect of the present invention, there is provided a method
for passivating a
metallic surface, said method comprising:
- providing said metallic surface;
- exposing said metallic surface to a composition comprising:
- a compound comprising an amine moiety;
- a compound comprising a sulfonic acid moiety; and
- a peroxide;
for a period of time sufficient to coat said metallic surface with a metal
oxide film, wherein the acid
is present in an amount ranging from 40 to 80 wt% of the total weight of the
composition and where
the peroxide is present in an amount ranging from 10 to 40 wt% of the total
weight of the
composition. Preferably, the sulfuric acid and said a compound comprising an
amine moiety and
said compound comprising a sulfonic acid moiety are present in a molar ratio
of no less than 1:1:1.
sulfuric acid, said compound comprising an amine moiety and said compound
comprising a
sulfonic acid moiety are present in a molar ratio ranging from 28:1:1 to
2:1:1.
7
Date Regue/Date Received 2022-08-12
According to a preferred embodiment of the present invention, the compound
comprising an amine
moiety has a molecular weight below 300 g/mol.
According to a preferred embodiment of the present invention, the compound
comprising a sulfonic
acid moiety is selected from the group consisting of: alkylsulfonic acids and
combinations thereof.
Preferably, said alkylsulfonic acid is selected from the group consisting of:
alkylsulfonic acids where the
alkyl groups range from C1-C6 and are linear or branched; and combinations
thereof.
DETAILED DESCRIPTION OF THE INVENTION
It will be appreciated that numerous specific details have been provided for a
thorough
understanding of the exemplary embodiments described herein. However, it will
be understood by those of
ordinary skill in the art that the embodiments described herein may be
practiced without these specific
details. In other instances, well-known methods, procedures and components
have not been described in
detail so as not to obscure the embodiments described herein. Furthermore,
this description is not to be
considered so that it may limit the scope of the embodiments described herein
in any way, but rather as
merely describing the implementation of the various embodiments described
herein.
According to a preferred embodiment of the present invention, there is
provided a method for the
passivation of a steel surface, wherein said method comprises the steps of:
- providing said steel surface to be passivated;
- cleaning said steel surface to remove oil and other contaminants;
- exposing said steel surface to a modified Caro's acid for a period of time
sufficient to form a layer
of metal oxide thereon, wherein said modified Caro's acid composition selected
from the group consisting
of: composition A; composition B and Composition C;
wherein said composition A comprises:
- sulfuric acid in an amount ranging from 20 to 70 wt% of the total weight of
the
composition;
- a compound comprising an amine moiety and a sulfonic acid moiety selected
from the group consisting of: taurine; taurine derivatives; and taurine-
related
compounds; and
- a peroxide;
wherein said composition B comprises:
- an alkylsulfonic acid; and
8
Date Regue/Date Received 2022-08-12
- a peroxide; wherein the acid is present in an amount ranging from 40 to 80
wt%
of the total weight of the composition and where the peroxide is present in an
amount
ranging from 10 to 40 wt% of the total weight of the composition;
wherein said composition C comprises:
- sulfuric acid;
- a compound comprising an amine moiety;
- a compound comprising a sulfonic acid moiety; and
- a peroxide;
for a period of time sufficient to coat said metallic surface with a metal
oxide film created by the exposure
of said metallic surface to said modified Caro's acid composition.
According to a preferred embodiment of the present invention, said sulfuric
acid, said compound
comprising an amine moiety and a sulfonic acid moiety and said peroxide are
present in a molar ratio of no
less than 1:1:1. Preferably, said sulfuric acid, said compound comprising an
amine moiety and a sulfonic
acid moiety and said peroxide are present in a molar ratio of no more than
15:1:1. Preferably also, said
sulfuric acid and said compound comprising an amine moiety and a sulfonic acid
moiety are present in a
molar ratio of no less than 3:1.
According to a preferred embodiment of the present invention, said compound
comprising an amine
moiety and a sulfonic acid moiety is selected from the group consisting of:
taurine; taurine derivatives; and
taurine-related compounds. Preferably, said taurine derivative or taurine-
related compound is selected from
the group consisting of: taurolidine; taurocholic acid; tauroselcholic acid;
tauromustine; 5-
taurinomethyluridine and 5-taurinomethy1-2-thiouridine; homotaurine
(tramiprosate); acamprosate; and
taurates; as well as aminoalkylsulfonic acids where the alkyl is selected from
the group consisting of CI-Cs
linear alkyl and CI-Cs branched alkyl. Preferably, said linear
alkylaminosulfonic acid is selected from the
group consisting of: methyl; ethyl (taurine); propyl; and butyl. Preferably,
said branched
aminoalkylsulfonic acid is selected from the group consisting of: isopropyl;
isobutyl; and isopentyl. Most
preferably, said compound comprising an amine moiety and a sulfonic acid
moiety is taurine.
According to a preferred embodiment of the present invention, said compound
comprising an amine
moiety is an alkanolamine is selected from the group consisting of:
monoethanolamine; diethanolamine;
triethanolamine; and combinations thereof.
According to a preferred embodiment of the present invention, said compound
comprising a
sulfonic acid moiety is selected from the group consisting of: alkylsulfonic
acids and combinations thereof.
9
Date Regue/Date Received 2022-08-12
Preferably, said alkylsulfonic acid is selected from the group consisting of:
alkylsulfonic acids where the
alkyl groups range from C1-C6 and are linear or branched; and combinations
thereof. More preferably, said
alkylsulfonic acid is selected from the group consisting of: methanesulfonic
acid; ethanesulfonic acid;
propanesulfonic acid; 2-propanesulfonic acid; isobutylsulfonic acid; t-
butylsulfonic acid; butanesulfonic
acid; iso-pentylsulfonic acid; t-pentylsulfonic acid; pentanesulfonic acid; t-
butylhexanesulfonic acid; and
combinations thereof. More preferably, said compound comprising a sulfonic
acid moiety is
methanesulfonic acid.
According to a preferred embodiment of the present invention, there is
provided a method for the
passivation of a steel surface, wherein said method comprises the steps of:
- providing said steel surface to be passivated;
- cleaning said steel surface to remove oil and other contaminants;
- exposing said steel surface to a modified Caro's acid for a period of time
sufficient to form a layer
of metal oxide thereon, wherein said modified Caro's acid composition selected
from the group consisting
of: composition A; composition B and Composition C;
wherein said composition A comprises:
- sulfuric acid in an amount ranging from 20 to 70 wt% of the total weight of
the
composition;
- a compound comprising an amine moiety and a sulfonic acid moiety selected
from the group consisting of: taurine; taurine derivatives; and taurine-
related
compounds; and
- a peroxide;
wherein said composition B comprises:
- an alkylsulfonic acid; and
- a peroxide; wherein the acid is present in an amount ranging from 40 to 80
wt%
of the total weight of the composition and where the peroxide is present in an
amount
ranging from 10 to 40 wt% of the total weight of the composition;
wherein said composition C comprises:
- sulfuric acid;
- a compound comprising an amine moiety;
- a compound comprising a sulfonic acid moiety; and
- a peroxide;
for a period of time sufficient to coat said metallic surface with a metal
oxide film created by the exposure
of said metallic surface to said modified Caro's acid composition.
Date Regue/Date Received 2022-08-12
According to a preferred embodiment of the present invention, said
alkylsulfonic acid; and said
peroxide are present in a molar ratio of no less than 1:1.
According to a preferred embodiment of the present invention, in Composition
C, said sulfuric acid
and said a compound comprising an amine moiety and said compound comprising a
sulfonic acid moiety
are present in a molar ratio of no less than 1:1:1.
According to a preferred embodiment of the present invention, in Composition
C, said sulfuric acid,
said compound comprising an amine moiety and said compound comprising a
sulfonic acid moiety are
present in a molar ratio ranging from 28:1:1 to 2:1:1.
According to a preferred embodiment of the present invention, the metallic
surface to be passivated
is stainless steel.
According to a preferred embodiment of the present invention, the metallic
surface to be passivated
is a stainless steel selected from the group consisting of: Austenitic;
Martensitic-PH; Ferritic; Martensitic;
Austenitic-FM; Ferritic-FM; and Martensitic-FM.
According to a preferred embodiment of the present invention, the metallic
surface to be passivated
is Austenitic stainless steel, having a chromium content ranging from 15.0 to
23.5 %.
According to a preferred embodiment of the present invention, the metallic
surface to be passivated
is Austenitic stainless steel and is selected from the group consisting of:
Type 304/304L; Type 316/316L;
Type 305; Custom Flo 302HQ.
According to a preferred embodiment of the present invention, the metallic
surface to be passivated
is a Martensitic-PH stainless steel having a chromium content ranging from
11.0 to 17.5 %.
According to a preferred embodiment of the present invention, the metallic
surface to be passivated
is Martensitic-PH stainless steel and is selected from the group consisting
of: Custom 630 (17Cr-4Ni);
Custom 450; Custom 455; Custom 465T; and 15Cr-5Ni.
11
Date Regue/Date Received 2022-08-12
According to a preferred embodiment of the present invention, the metallic
surface to be passivated
is a Martensitic stainless steel, having a chromium content of less than 15 %.
According to a preferred embodiment of the present invention, the metallic
surface to be passivated
is Martensitic stainless steel and is selected from the group consisting of:
Type 410; Type 420; and
TrimRite .
According to a preferred embodiment of the present invention, the metallic
surface to be passivated
is a Ferritic stainless steel having a chromium content above 16 %.
Preferably, said Ferritic stainless steel
is type 430.
According to a preferred embodiment of the present invention, the metallic
surface to be passivated
is a Ferritic stainless steel having a chromium content of less than 12 %.
Preferably, said Ferritic stainless
steel is type 409.
According to a preferred embodiment of the present invention, the metallic
surface to be passivated
is a Austenitic-FM stainless steel having a chromium content ranging from 17
to 19 %. Preferably, said
Austenitic-FM stainless steel is type 303.
According to a preferred embodiment of the present invention, the metallic
surface to be passivated
is a Ferritic-FM stainless steel having a chromium content above 16 %.
Preferably, said Ferritic-FM
stainless steel is selected from the group consisting of: type 430F; 430FR;
and Chrome Core 18-FM.
According to a preferred embodiment of the present invention, the metallic
surface to be passivated
is a Ferritic-FM stainless steel having a chromium content of less than 13 %.
Preferably, said Ferritic-FM
stainless steel is type 409Cb-FM.
According to a preferred embodiment of the present invention, the metallic
surface to be passivated
is a Martensitic-FM stainless steel having a chromium content of less than 13
%. Preferably, said
Martensitic-FM stainless steel is type 416.
Conventional nitric acid baths are changed when, upon titration, it is deemed
that the solution is no
longer capable of providing proper passivation. Another indicator used to
decide whether to discard the
acidic solution is by visual inspection and assessing the color of the bath.
12
Date Regue/Date Received 2022-08-12
According to a preferred embodiment of the present invention, the passivation
bath using a
modified Caro's acid, is able to process more metal than a conventional
passivation bath. Moreover, it can
extend its useful lifetime (i.e., period of time until the bath chemicals need
to be discarded and replaced)
by adding the peroxide component at intervals where the bath is known to start
to lose its passivation
efficiency.
Experiments to passivate various steels
Various steel coupons were exposed to a composition comprising a modified
Caro's acid as set out
previously in order to assess the resulting passivated coupon's ability to
withstand corrosion through acid
exposure.
Corrosion of a Modified Caro's acid to treat Stainless Steel
The purpose of this experiment is to test the corrosion rates of stainless
steel in the presence of a
modified Caro's acid and the corrosion rates of stainless steel after exposure
to the delignification blend.
Procedure
Corrosion tests were completed in glass sample jars in a heated water bath.
For each condition
listed in Table 1, the coupon was washed with acetone, air dried, and weighed,
before being suspended in
the test fluid. The fluid in each glass sample jar was pre-heated to
temperature before exposing the coupon
to the acid blend. After the exposure period, the coupon was removed, washed
with water, followed by an
acetone wash, air dried, and then weighed. The corrosion rate was determined
from the weight loss, and the
pitting index (Appendix A) was evaluated visually at 40X magnification, and a
photo of the coupon surface
at 40X magnification was taken.
Table 1 lists the various compositions which were tested in the corrosion
testing. The corrosion
testing was carried out for a duration of 24 hours at a temperature of 30 C
(86 F) under atmospheric
pressure.
Table 1: Listing of the compositions and coupons tested
Test Coupon type Test fluid
A 316SS 40 wt% sulfuric acid
B-1 304SS Delignification blend
C-1 316SS Delignification blend
B-2 304SS 70 wt% sulfuric acid
C-2 316SS 70 wt% sulfuric acid
13
Date Regue/Date Received 2022-08-12
The corrosion test results are shown Table 2, and in Figure 1 to Figure 2.
Figure 1: Surface of
304SS coupon from corrosion test B-2 at 40X Magnification. Figure 2: Surface
of 316SS coupon from
corrosion test C-2 at 40X Magnification.
Table 2: Results of the corrosion testing of treated and untreated coupons
at 30oC under
atmospheric pressure for a duration of 24 hours
Test Coupon # Corrosion rate Pitting index
mm/year lb/ft2
A 325 40.756 0.180 9
B-1 19 0.014 0.000 0
C-1 310 0.000 0.000 0
B-2 19 0.002 0.000 0
C-2 310 0.007 0.000 0
Pitting index scale found in Finggar, M.; Jackson, J. Corrosion Science, 2014,
86, 17-41
Conclusion
From the above experiments, it was found at the tested conditions that the
modified Caro's acid
composition was not corrosive to 304SS and 316SS (coupons B-1 and C-1).
Moreover, exposure of coupons
B-1 and C-1 to a modified Caro's acid according to a preferred method of the
present invention clearly
passivated their surfaces. This is confirmed by subsequent exposure of those
coupons to 70 wt% }12504,
which resulted in an unblemished surface (as indicated by the photographs
taken of the coupons now
labelled B-2 and C-2.
Moreover, after the exposure to the modified Caro's acid, the steel coupons of
304SS and 316SS
were resistant to sulfuric acid corrosion at the tested conditions, whereas
coupons not exposed to the
modified Caro's acid were heavily corroded by sulfuric acid.
According to a preferred embodiment of the present invention, the compositions
used for
passivation of metals such as stainless steel can be reused be simply adding
more peroxide. When used for
passivating metals, the compositions consume the peroxide component and thus
can be re-used a great
number of times without losing effectiveness, as the consumable peroxide can
be replenished by simply
adding it to the used composition. This is a significant advantage over
standard nitric acid and citric acid
solutions, as they lose effectiveness over time and must be disposed of
entirely.
14
Date Regue/Date Received 2022-08-12
According to a preferred embodiment of the present invention, the passivation
method is employed
to replace passivation by citric acid and treats metals used in a wide variety
of industries, including but not
limited to aerospace, medical, industrial and manufacturing industries.
While the foregoing invention has been described in some detail for purposes
of clarity and
understanding, it will be appreciated by those skilled in the relevant arts,
once they have been made familiar
with this disclosure that various changes in form and detail can be made
without departing from the true
scope of the invention in the appended claims.
Date Regue/Date Received 2022-08-12
