Note: Descriptions are shown in the official language in which they were submitted.
9~
The present invention relates to a method for the
production of an extremely thin tin plated steel sheet possess-
ing excellent weldability charac-teristics. More particularly,
the invention relates to a method for the production of a steel
sheet having an upper layer of a thin hydrated chromium oxide
and a lower thin layer of tin.
Recently, in the field of food cans, a change has
rapidly taken place from expensive electro-tin plates to
cheaper tin free steel (TFS) consisting of a metallic chromium
and hydrated chromium oxide. It has also been observed that
there is a rapid decrease in the weight of the tin coating in
electro-tin plates used to manufacture food cans. This is
because the tin used for the production of tin plates is very
expensive, and there is some concern over the exhaustion of
tin resources in the world.
An ordinary metal can consists of two can ends and
a single can body. In the case of I'FS, the seaming of the can
body is generally carried out with nylon adhesives by using
the Toyo Seam and Mira Seam rnethods.
Nowaclays, a ~FS can body seamed by a nylon adhesive
is not only used for beer and carbonated beverages, but is also
used for foods, such as fruit juices (which are immediately
placed after pasteurization at a temperature of 90 - 100C),
or coffee, meat and fish ~which are pasteurized by hot steam
at a temperature above 100C in a retort after being packed in
the can at 90 - 100C)~ This is the result of the progress in
the TFS manufacturing techniques and improvements of TFS films
as exemplified in Laid-Open Japanese Patent Applications No.
Sho 53-58442, No. Sho 54-64034 and No. ShO 54-89946.
Laid-Open Japanese Patent Application No. Sho 53-
58442, inventors: T. Nishimura, J. Morita, K. Yoshida,
Y. Tsukahara and K. Koyama, entitled "Process of Producing
,~ - 1 -
~7~
Electrolytic Chromic Acid Trea~ed Steel Sheet", Applicant:
Japan Steel Corporation, Date of Application: 6th November,
1976, relates to the electrolyte used in producing tin free
steel having a lower layer of metallic chromium and an upper
layer of hydrated chromium oxlde. After metallic chromium
plating, the steel sheet is electrolysed in a chromic acid
solution in which the concentration of SO4 is 0.01 ~ 0.7%
of CrO3. By the above-mentioned process, one can obtain tin
free steel for cemented can body having small reduction of
peel strength of adhered part aged in hot water.
Laid-Open Japanese Patent Application No. Sho
54-64034, lnventors: T. Inui, H. Kuroda, K. Hizuka and ~.
Hamano, Applicant: Toyo Kohan Co., Ltd., entitled "Pretreatment
of Electrolytic Chromic Acid Treated Steel Sheet", Date of
Application: 31st October 1977, relates to the pretreating
of tin free steel. Prior to the cathodic electrolysis, steel
sheet is subjected to an anodic electrolysis followed by
cathodic electrolysis in solu~ion consisting of one or more
than two kinds of chromic acid, alkaline metal, chromate or
dichromate, ammonium chromate and ammonium dichromate after
degreasing and pickling~ This process enable~ to obtain tin
free steel for cemented can body having small reduction of
peel strength of adhered part after aging in hot water.
Laid-Open Japanese Patent Application No. Sho 54-
89946, inventors: M. Kamata, M. Higuchi, Y. Tsukamoto and K.
Tano, Applicant: Japan Steel Corporation, entitled "Process
of Producing Electrolytically Chromated Steel Sheet Having
Small Reduction of Peel Strength After Aging", Date of
Application: 28th December, 1977, relates to the post treating
process of electrolytically chromated steel sheet having a
lower layer of metallic chromium and an upper layer of
hydrated chromium oxide produced by cathodic e:Lectrolysis in
~lt~
chromic acid, chromate and dichromate solution. Af~er
metallic chromium plating and rinsing, the s~eel sheet is
treated in hot water at 65 ~100C, a pFI more than 4 during
0.5~,15 seconds. One can obtain tin free steel for cemented
can body having small reduction of peel strength of adhered
part after aging in hot water.
Another method of seaming a TFS can body by electric
welding is also well known. In such electric welding process,
however, the seaming process is intricate because the metallic
chromiurn layer and the hydrated chromium oxide layer must be
mechanically or chemically removed from the TFS surface.
On the other hand, the seaming of a tin plate can
body is generally carried out by soldering. In this soldering
process, however, it is impossible to decrease the weight of
the tin coating on the tin plate to under 2.8 g/m2, because
it is difficult to stabilize the soldering process when the
weight of the coating is under 2.8 g/m .
A method oî seaming the tin plate can body employing
organic adhesives has been also proposed, for in~stance in
Laid-Open ~apanese Patent Application No. Sho 49~3782 9 and
Japanese Patent Publiaation No. 5ho 48-18929,
Laid-Open Japanese Patent ~pplication No. Sho 49-
3782 9, inventors: H. Eotta and T. Mori, Applicant: K. Yoshizaki,
entitled "Tin Plated Steel Sheet Having Excellent Adhesive
Strength", Date of Application: 11th August, 1972, relates to
tin plated steel sheet for cemented can body having high ad-
hesive strength, The surface of tin plated steel consists of
metallic tin and iron-tin alloy of which the area of exposed
iron-tin alloy is more than 30% of the total surface area. One
can obtain tin plated steel sheet for cemented can having high
adhesive strength using the above-mentioned tin plated steel
sheet.
-- 3 --
11'7~
` Japanese Patent Publication No. Sho 48-18929,
inventor: H. ~Io-tta, AppLicant. K. Yoshizaki, entitled
"Process for Adhering Tin Plated Steel Sheet with Organic
Adhesive", Date of Application: 4th December, 1969, relates
to a process for producing a cemented can body using a tin
plated steel sheet and a linear polyamide having a lower
melting point than that of tin after base coating and curing.
The tin plate used has a hydrated chromium oxide layex of
3 ~20 ~g/dm2 and a tin oxide layer of 160 ~640 mc/dm2 equi-
valent to the amount of electricity required for the reduct-
ion of tin oxide to tin, and a base coating lacquer made of
20 ~40 parts of resol type phenolic resin and 80~ 60 parts of
epoxy resin. This process enables to obtain a cemented can
body having high peel strength more than 20 kg/cm.
However, after a few months, the tin plate can body
seamed by an organic adhesive may be broken, because the b~nd-
ing strength in the seam becomes quite low.
In view of t'he above, a lap seam welding, for in-
stance, one obtained by the SoudronicrM process 'has recently
been proposed as a new met'hod of seaming the tin plate can body.
The Soudronic process involves the continuous production of cans
by means of electrically resistant seam welding. After rounding
the blank sheet, the overlapping ends are joined by electrical
heating using alternating current between two copper wires
serving as electrodes. This Soudronic process is used for
manufacturing an aerosol can body or a dry fill can body. In
this field, it is also desirable to decrease the tin coating
weight, but with a decrease of the tin coating weight, the weld-
ability of the tin plate becomes poor.
It is an object of the present invention to provide
a method for the production of an extremely thin tin plated
~teel sheet having excellent weldab:ility as well as excellent
_ a,
~t7~ 8
lacquer adhesion and corrosion resistance after lac~uering and
forming.
This object can be accomplished by the formation of
a uniform hydrated chromium oxide layer containing from 0.5 to
5 mg/m , calculated as chromiurn, on a steel sheet which has
been covered with a dense and thin layer of tin in which the
tin coating weight is 0.05 - 1.12 g/m .
It is another objec~ of the present invention to
provide a process for producing a stee1 sheet having a lower
layer of tin and an upper layer of hydrated chromium oxide, said
steel sheet having excellent weldability, which comprises
electrolytically tin plating a substantially clean steel sheet
in an electrolyte containing a compound selected from the group
consisting of stannsus sulfate, stannous phenolsulfonate,
stannous chloride, stannous fluoborate, sodium stannate and
potassium stannate, to obtain a tin plated steel sheet having
an amount of plated tin of 0.05 - 1.12 g/m2, and subjecting
the resultant steel sheet to a treatment in a solution con-
taining hexavalent chromium ions, or hexavalent chromium ions
and at l.east one member selected from the g:roup consisting of
a trivalent chromium compound, a sul-fur compound, a fluorine
compound and a pho~phorus compound to form an upper layer
- consisting essentially of hydrated chromium oxide in an amount
of 0.5 - 5 mg/m , calculated as chromium.
As a result of investigations on the weldability of
extrernely thin tin plated steel sheets having a hydrated
chromium oxide layer fonned by treatment of the tin coated
steel sheet in a solution containing mainly hexavalent chromi~un
ions, it has been determined that the ~eldability of said tin
.plated steel sheet by the Soudronic process depends on the
amount of hydrated chromi~m oxide, calculated as chromiuml on
said thin tin plated steel sheet. The weldability is improved
~7'3~
with a decrease of the amount o~ hydrated chromium oxide, as
chromiu~, particularly in the case of a thin tin plated steel
-- sheet.
It is assumed that the effect of the arnount of
hydrated chromium oxide, based on the amount of chromium, can
be neglected in the case when the tin coating weight is in-
creased, because a large amount of tin which is present under
- the hydrated chromium oxide layer is melted during welding, and
in the case of a weight decrease of the tin coating, the amount
of hydrated chromium oxide as-chromium affects the weldability,
because the thin tin layer is immediately converted into an
iron-tin alloy layer.
As described above, by simply decreasing the amount
of hydrated chromium oxide, as chromium, on the -thin tin
plated steel sheet, the w~ldability is improved, but the
desired excellent lacquer adhesion and corrosion resistance,
after lacquering, and forming, are not maintained. So, it is
also important in the present invention to subject the steel
sheet to a dense and khin tin plating step, which is necessary
for the format:ion of a uniform and thin hydrated chromium
oxide layer on said thin tin plated steel sheet.
In the present invention, a method for the production
of a thin tin plated steel sheet comprises, as the only essen-
tial steps, electrolytically tin plating a substantially clean
steel sheet and subjecting the resultant steel sheet to chromic
acid treatment to form a hydrated chromium oxide layer on the
exposed surface of tin. From an industrial point of view, the
present inve~tion can be carried out according to the follow-
ing process: degreasing with an alkali and pickling with an
acid -~ water rlnsing ~ very thin electrolytic tin plating --
~water rinsing --~ chromic acid treatment ~ water rinsing_-
~drying.
~; - 5a -
~1~7~
In some cas~s, the heating of the very thin tin
plated steel sheet at a temperature below or abo~e the meltiny
point of the tin and the subsequent quenching operations are
carried out after the thin tin plating step.
To carry out the electrolytic tin plating according
to the present invention, a known tin plating electrolyte may
be e~ployed, for exarnple, an acidic electrolyte, such as
- stannous sulfate, a stannous aromatic sulfonate, stannous
fluoborate~and stannous chIoride, an alkaline electrolyte,
such as sodium stannate and potassium stannate; or a neutral
electrolyte such as stannous sul~ate containing a carboxylic
acid additive'may be employed. For the formation of a dense
tin layer according to the present invention it is preferable
to use the known alkaline electrolytes or the weakly acidic
electrolytes having a low concentration of stannous ions
(described in Japanese Patent Application ~o. Sho 46-25603,
Laid-Open Japane~e Patenk Application ~. Sho 55-73887),
especially an improved alkaline electrolyte (describ~d in Laid-
Open Japanese Patent Application No~ Sho 54-117332), in which
a considerable arnount of hydrogen gas is generated.
Japanese Patent Publication ~o. Sho 46-25603,
.inventors: T. O~ama, A. Miyachi and T~ Kikuta, Applicant:
Toyo Kohan Co., Ltd., entitled "Acidic Tin Preplating Process",
Date of Application: l9th April, 1967, relates to a process
of preplating tin in acidic solution. Prior to tin plating in
acidic solution, preplaking is carried out in acidic solution
containing stannous ion of 1.6 ~ 15 g~l and acid equivalent to
sulfuric acid of 1 ~ 9.5 g/l. Thi.s process enables to obtain a
tin plat~ having excellent corrosion resistance.
Laid-Open Japanese Patent Application Nc. Sho 55-
73887, inventors: T. Inui, H. Kuroda, T. Hanafusa and K.
Yazaki, Applicant: Toyo Kohan Co., Ltd~, entitled "Acidic
- 5b -
~ 17 ~.3~ ~
Tin Plating Bath, Dake of Application: 27th November, 1978,
relates to a tin plating bath consisting of surface or
phenolic sul~onate bath containing 1.5~50 g/l of stannous
ion' the bath contains one or more than two kinds of sulfate
of alkaline metal, aluminum, ammonium, manganese and chrom-
ium in the range of 5 ~150 g/l. This acidic bath enables
to obtain fine and dense deposition of tin.
Laid-Open Japanese Patent Application No. Sho
54-117332, inventors: T. Inui, H. Kuroda, F. Kunishige and
K. Hakota, Applicant: Toyo Kohan Co., Ltd.r entitled "Alkaline
Tin Plating Bath, Date of Application: 6th March, 1978, relates
to a tin plating bath consisting of stannate and hydroxide of
alkaline metal; the bath contains one or more than two kinds of
aluminate, borate, molybdate, tungstate, chromate, dichromate
of alkaline metal or ammoniu~. With the above~mentioned
alkaline bath, one can obtain fine and dense depositîon of tin
and also fine and dense iron-tin alloy a~ter heating. The
lacquer adhesion and corrosion resistance of the tin plate
using the bath described in this invention are higher than
that o~ tin plate without additives.
According to the present invention, the following
electrolytic tin plating conditions are employed when an
acidic electrolyte is used:
Concentration of stannous ions: 1~5 - 15 g/l
Concentration o~ free acid (as
~ H2SO4): 1.0 - 15 g/l
Concentration o~ organic addition
agents, such as ethoxylated
~-naphthol sulfonic acid or
cresol sulfonic acid: 1 - 6 g/l
Temperature o~ the electrolyte: 25 60C
Current density: 5 - 50 A~dm
_ 5c -
1~7~3~2~
Generally, a lower current density is used ~or the
formation of a dense tin layer at lower electrolyte temperatures,
and for a lower concentration of the stannous ions. Finally,
lower current densities are also used for higher concentrations
of the free acid. On the contrary, when higher temperatures
are employed and if there is a higher concentration of the
stannous ions as well as lower concentrations of free acid, a
- higher current density must be applied. Furthermore, when the
concentration of the stannous ions and of the free acid are
below 1.5 and 1.0 g/l, respectively, the electrical resistance
of the electrolyte increases and the current efficiency during
- 5d -
the tin plating operation becomes very low, and therefore, such
low concentrations are not suitable for the industrial produc-
tion of thin tin plated steel sheets.
According to the present invention the electro-tin
plating is preferably carried out at a current efficiency of
20 - 70 percent in respect to the tin deposition.
According to the present invention, the following
electrolytic conditions are employedl when alkaline electro-
lytes are used:
Concentration of stannic ions: 30 ~ 70 g/l
Concentration of base (as NaOH or KOH): 10 - 25 g/1
Temperature of the electrolyte: 70 - 90C
Current density: 1 - 10 A/dm2
Generally, in an alkaline electrolyte, as compared
with an acidic electrolyte, a more dense tin layer is obtained
but the current e~ficiency of the tin plating step is lower.
Especially, the current efficiency for tin platiny decreases
remarkably with an increase in the current density and a de-
crease in the ternperatur~ of the electrolyte.
In the present invention, the optimurn range for the
amount of plated tin is from 0.05 to 1~12 g/rn2, preferably from
0.22 to 1.12 g/m2. If the amount of tin is less than 0.05 g/m2,
- the corrosion resistance becomes quite poor. An increase in
the amount of tin to above 1.12 g/m2 is not economical because
of the high price of tin, although the weldability would not
be affected.
In some cases, the thin tin plated steel sheet is
heated at a temperature above or below the melting temperature
o~ the tin and the thus heated plate is quenched. In this case,
heating is carried out by kno~l methods such as resistance
heating as is generally used in the electro-tin plating.
The hydrated chrorni~n oxide layer is forrned on the
9~
~hin tin pla-ted steel sheet, according to a cathodic treatmént
or an i~nersion treatment in a known solution containing hexa-
valent chromium ions, such as a sodium dichromate solution or a
chromic acid solution.
In the present invention, the hexavalent chromium
compound is selected from the group consisting of chromic acid,
ammonium chromate, ammonium dichromate, and the chromate or
dichromate of an alkali metal. It is also possible in some
cases to add at least one compound selected from trivalent
chromium compounds, sulfur compounds, fluorine compounds and
phosphorus compounds to the hexavalent chromium ion-containing
solution.
Trivalent chromium ions are formed by the addition of
a trivalent chromium compound, such as chromium sulfate, chrom-
ium hydroxide, by the addition of a reducing agent such as an
alcohol and hydrogen peroxide or by an electrolytic reduction
of the hexavalent chromium ions. The sulfur compounds are
selected from the group consisting of sulfuric acid, an aromatic
disulfonic acid, a sulfate, a sul~ite, a thiocyanate, or an
aromatic disulfonate of a~nonium and alkali metals and thiourea.
The fluorine compound is selected from the group consisting of
hydrofluoric acid, a hydrofluoboric acid, a hydrofluosilicic
acid, a fluoride, a borofluoride, or a silicofluoride of
ammonium and alkali metals. The phosphorus compound is select-
ed from the group o~ phosphoric acid, pyro-phosphoric acid, a
phosphate, or a pyrophosphate of ammonium and alkali metals.
In the case of an immersion treatmen-t of the thin
tin plated steel sheet, it is desirable to control the pH of
the solution below 6, and to provide for an irnmerslon time of
about 0.1 - 10 seconds.
In the case of an electrolytic treatment, the thin
tin plated steel sheet is generally subjected to a cathodic
~7~28
treatment using an aqueous solution containing hexavalent
chromium ions.
However, according to the present inven~ion, an anod
ic treatment, an anodic treatment after a cathodlc treatment
or a cathodic treatment after an anodic treatment is also
applied for the treatment of the thin tin plated st,eel sheet.
Furthermore, it is poss.ible to repeat these treatments several
times. As a matter of practical application, a quantity of
electricity below 10 coulombs/dm2 is sufficient for the forma-
tion of the hydrated chromium oxide having less then 5 mg/m2
as chromium on the thin tin plated steel sheet, although the
quantity of electricity depends on the composition of the elec-
trolyte, the pH of the electrolyte, the temperature of the
electrolyte and the sur-face conditions of the tin plated steel
sheet.
The conditions for the formation of the hydrated
chromiurn oxide are surnmarized as f~llows:
Concentration of hexavalent chromium .ion~: 5 - 50 g/l
pH of solution: 1 - 6
Temperature of ~olution: 35 - 60C
Current density (in case of electrolytic
treatment~: 5 - 50 A/dm2
Treating time or immersion time: 0.1 - 10 sec.
Concentration of additives such as sulfur compound,
fluorine compound and phosphorous compound in
some cases: 1/10 - 1jl50 of the hexavalent
chromium ions
The optimum range for the amount of hydrated chromium
oxide formed under the conditions described above is 0~5 - 5
mg/m2, preferably 0.5 - 3 rng/m2, calculated as chromium. If
the amount of hydrated chromium oxide calculated as chromium,
is above 5 mg/m2, the weldability is decreased.
~1'7~
Generally, the weldabllity is i~proved when the
amount of hydrated chromium oxide is decreased because the sur-
face resistance is decreased, but if the amount of hydrated
chromium oxide falls below 0.5 mg/m2, based upon the calculated
chromium content, the corrosion resistance and the lacquer
adhesion is substantially decreased.
It is very important according to the present inven-
tion that a thln and uniform hydrated chromium oxide layer be
formed on the thin tin plated steel sheet in order to achieve
all the properties including excellent weldability, corrosion
resistance after lacquering, forming as well as excellent
lacquer adhesion. It has been found that the uniformity of
the formed hydrated chromium oxide layer depends on the uni-
formity and denseness of the tin layer to be trea-ted, although
it is also affected by the conditions used in the formation
of the hydrated chromium oxide. I~lat is to say, in the case
when the plated tin layer does not sufficiently cover the sur-
face of the steel sheet, the hydrated chromium oxide layer
formed on tin plated steel sheet is not uniform and shows a
network like structure~
Therefore, it is very :important according to the
present invention that the steel sheet be subjected to a dense
and uniform tin plating by using the known alkaline electro
lytes or the weakly acidic electrolytes having a low concentra-
tion of stannous ions in order to form a uniform hydrated
chromium oxide layer on the thin tin plated steel sheet.
This invention is illustrated by the following
examples.
EXAMPLE 1
A cold reduced steel sheet having a thickness of 0.23
mm was electrolytically degreased in a solution of sodium
hydroxide and then pickled in dilute sulfuric acid. The steel
~ ~'f'~3 ~ ~ ~
sheet, after beiny rinsed with water, was electroplated with
tin under the ~ollowing plating conditions and then rinsed
with water and dried.
Composition of electrolyte:
Stannous sulfate: 5 g/l
Phenolsulfonic acid (60% a~ueous
solution): 20 g/l
Ethoxylated ~-naphthol sulfonic acid: 5 g/l
Bath temperature: 40C
Cathodic current density: 10 A/dm2
Tin coating weight: 0.51 g/m2
After that, the tin on the steel sheet was melted by
using resistance heating, and then the steel sheet was treated
by immersiny it into the following solution:
Composition of solution:
Chromic acid: 25 g/l
Sodiwm hydroxide: 10 g/l
Bath temperature: 40~C
Chromium weight in hydrated chromium oxide: 1.4 mg/m2
XAMPLE 2
A steel sheet pre treated as in Example 1 was elec-
troplated with tin under the following plating conditions after
which it was rinsed with water and dried:
Composition of electrolyte:
Sodium stannate: 80 g/l
Sodium hydroxide: 20 g/l
Bath temperature: 85~C
Cathodic current density: 15 A/dm2
Tin coating weight: 0.39 y/m2
After that, the tin coated steel sheet w~s heated at
210C, and then was treated by immersing it into the following
solution:
-- 10 --
3~
Composition of solution:
Chromic acid: 30 g/l
Sodium fluoride: 0.5 g/l
Bath temperature: 50C
Chromium weight in hydrated chromi~n oxide: 0.7 mg/m2
EXAMPLE_3
A steel sheet pre~treated as in Example l was elec-
troplated with tin under the following plating conditions
after which it was rinsed with water and dried:
lOComposition of electrolyte:
, Sodium stannate: 70 g/l
Sodium hydroxide: 15 g/l
Sodium aluminate: 40 g/l
Bath temperature: 85C
; ~athodic current density: 5 A/dm2
~in coating weight: ~-24 g/m2
Af-~er that, tin on the steel sheet was melted by
using resistance heating, and khen the steel sheet was cathod
ically treated under the following conditions, and rinsed with
water and dried:
Composikion of electrolyte:
Sodium dichromate: 30 g/l
Bath temperature~ 40~C
Cathodic current density: 5 A/drn
Chromium weight in hydrated chromium oxide: 2.3 mg/m2
COMPARATIVE ExAMæLE 1
A steel sheet pre-treated as in Example 1 was elec-
troplated with tin of 0.25 g/m2 under the same conditions as
in Example l. After rinsing with water, and drying, the tin on
the coated steel sheet was reflowed by using resi.stance heating
and then was cathodically treated under the same conditions as
in Exarn~le 1 for the formation of hydrated chromium oxide on
the tin coated steel sheet in an arnount of 6.1 mg/m2, calculat-
ed as chromium. After that, the treated steel sheet was rinsed
, - -~
-- 11 --
~ ~9~
with water and dried.
COMRARATIVE EXAMP~E 2
A steel sheet pre-treated as in Exarnple 1, was
electroplated with tin in an amount of 0~56 g/m2 under the
same conditions as in Example 1. After rinsing with waterl the
tin coated steel sheet was cathodically treated, undsr the
same conditions as in Example 1 for the formation of hydrated
chromium oxide on the tin layer in an amount of 8~2 mg/m~.
After that, the treated steel sheet was rinsed with water and
dried.
The weldability, corrosion resistance and lacquer
adhesion of the thus treaked steel sheet in the above described
Examples and Comparative E~arnples were evaluated by the follow-
ing testing methods, the results of which are shown in the
attached Table.
(1) Weldability
The weldability was evaluated by using a welding test
machine which is similar to the Soudronic type having a copper
wire as an intermediate electrode, under the following welding
conditions:
Welding conditions:
Power supply frequency: 60 Hz
- Welding speed: 8.4 m/min.
Overlap of sheet: 0.4 mm
Added pressure: 45 kg
The weldability was shown as an available ranye of
secondary current in welding~ The upper limit in the available
secondary current range corresponds to the welding conditions in
which some defect, such as splash, is found and the lower limit
corresponds to the welding conditions in which the breakage
occurs in the parent rnetal or welded part by tearing tests.
This weldability is judged by the method of Williams,
- 1~
that is to say, the wider the secondary current range in wel~-
ing, the better the weldability.
(2) Corrosion Resistance After Lacquerin~ and Forminq
The sample was based at 210~C far 12 minutes af-ter
coating with 50 mg/drn2 of an epoxy-phenolic type of lacquer.
The coated sar~ple was cut to a size of 15 mm x 100 mm. The
test piece was bent to 180 by the drop of a 3 kg weight from a
height o 150 mm, after placing a steel sheet having a thick-
ness of 0.25 mm between the pre-bent test piece. The bent
test piece was sealed with paraffin, except for the formed
part, and was put in 300 ml of a 0.01 mole/l phosphoric acid
solution at room temperature for one week.
The same procedure was repeated for another test
piece, except that a 0.01 mole/l citric acid solution contain-
ing 0.3 percent by weight of sodium chloride, was used. The
iron pick-up in each solution was measured.
(3) Lacquer Adhesion
Two pieces of the sample were pretreated. One piece
of the sample was baked at 210C for 12 minutes after coating
with 60 mg/dm2 of an epoxy-phenolic type of lacquer and the
other piece was baked under the same conditions as described
above after coating with 25 mg/dm2 of the sarne lacquer. The
two pieces having a different coating, were each cut to a size
of 5 mm x 100 mm, were bonded together by using a 100 ~n nylon
film at 200C for 30 seconds under 3 kg/cm2 of pressure by
using a hot press after preheating at 200C for 120 seconds.
The peel strength (kg/5 rMn) of the assembly was
measured by a conventional tensile testing machine.
The weldability, corrosion resistance after lacquer-
ing and forming and lacquer adhesion of the steel sheet obtainedby the Examples and the Comparative Exarnples were evaluated
by the tests described above, the results of which are shown
in the Table.
1~'7~
As it is apparent from the Table, the treated steel
sheet of the present invention has excellent weldability,
corrosion resistance after lacquering and forming, and excel-
lent lac~uer adhesion,
This treated steel sheet is therefore quite suitable
for use as a material for making welded cans.
;'
14 -
~1~'7~
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,Q O ~ h (D t~
~_1 tl) O s~ ~1 ~ r~
h ~r1 ~ ~ nl 0
1: h U~ _ U
h O S-l 1~1 E
O
- -- 15 --
