In-process formation of hard surface layer on Ti/Ti alloy having high resistance

A process for producing a titanium material with excellent corrosion resistance, which comprises first applying a degree of cold working of 10% or more of the total working reduction while causing an oil to exist on the surface of the titanium material during cold working thereof and then subjecting the titanium material to in-situ heat treatment at a temperature of 300.degree. C. or higher, thereby forming a layer with excellent corrosion resistance containing at least one of Ti.sub.2 N, TiC and Ti(CN) on the titanium material surface.

BACKGROUND OF THE INVENTION 
This invention relates to a process for producing a titanium material 
having a layer with excellent corrosion resistance formed on the surface. 
Titanium which itself has excellent corrosion resistance is being used in 
various field but has been used under increasingly severe corrosion 
environments in recent years, whereby there arise problems of general 
corrosion or crevice corrosion. 
For solving such problems, there is the method of using corrosion resistant 
titanium alloys such as Ti-Pd, and there is also known the method of 
improving corrosion resistance by a surface treatment of titanium. 
However, a corrosion resistant titanium alloy such as Ti-Pd has a drawback 
in that the cost becomes very high because an expensive noble metal is 
added. In the surface treatment methods, there have been developed the 
method in which palladium, ruthenium or oxide thereof is applied as a 
coating on the surface and the method in which titanium nitride or 
titanium carbide is bonded to the surface by ion plating or heat treatment 
in gases. However, in the former method, the cost becomes high because of 
the use of an expensive metal, while the latter method, which is 
specifically atmospheric annealing, requires troublesome steps and the 
heat treatment temperature exceeds the transformation point, whereby there 
is the problem of deterioration of the titanium material. 
The present invention has been accomplished in view of the above situation, 
and as a result of various studies on the surface treatment methods for 
improving corrosion resistance of titanium, the present inventors have 
found a process for producing a titanium material which is very simple and 
has remarkably increased corrosion resistance. 
Briefly, it has been found that the corrosion resistance of a titanium can 
be remarkably improved by permitting an oil to exist on the titanium 
surface at the time of cold working thereof, then causing the oil to 
adhere firmly onto the titanium surface by performing cold working and 
thereafter applying heat treatment at 300.degree. C. or higher 
temperature. 
Based on this discovery, the present invention is intended to provide a 
process for producing very simply and inexpensively a titanium material of 
excellent corrosion resistance. 
SUMMARY OF THE INVENTION 
According to the present invention there is provided a process for 
producing a titanium material of excellent corrosion resistance, which 
comprises, during cold working of a titanium material, subjecting the 
material to 10% or more of the total degree of cold working while 
permitting an oil to exist on the surface of the titanium material and 
then subjecting the titanium material to heat treatment at a temperature 
of 300.degree. C. or higher, thereby forming a layer having excellent 
corrosion resistance containing at least one of Ti.sub.2 N, TiC, Ti(CN) on 
the titanium material surface.

DETAILED DESCRIPTION OF THE INVENTION 
In the present invention, an oil is permitted to exist on the titanium 
surface during cold working because the active titanium surface generated 
during working is caused to react with the oil, and at the same time the 
oil is baked by the heat generated thereby, but corrosion resistance 
cannot be improved only with such treatment. By performing thereafter heat 
treatment at 300.degree. C. or higher temperature, the oil firmly adhering 
to the surface is decomposed to react with titanium to form a surface 
layer, which improves remarkably the corrosion resistance. 
In order to determine the nature of the mechanism in greater detail, the 
titanium surface resulting when pure titanium (Grade 2) was worked to a 
thickness of from 0.5 mm to 0.2 mm by cold rolling with the use of an oil 
for rolling and then subjected to heat treatment in an argon atmosphere at 
650.degree. C. for 3 hours was observed by SEM. The result is shown in the 
photograph in FIG. 4, in which it can be seen that the surface is not flat 
but there can be seen some places on which titanium turns to form 
so-called "scabs". Such scabs may be formed during rolling of active 
titanium through baking of titanium onto rolls heated to high temperature 
by the working heat or formation of unevenness by adherence of a part 
thereof again onto titanium, which is then extended by rolling to form 
scabs as seen in the photograph. When carbon analysis was conducted for 
the vicinity of the scab and the flat place by EPMA (electron probe micro 
analyzer), it was found that a great amount of carbon exists in the 
vicinity of the scab as compared with the flat portion as shown in FIG. 5. 
Thus, it was found that there are Ti(CN), TiC with high corrosion 
resistance in this portion along with the result of X-ray analysis as 
described below. 
From these results, we speculated the mechanism of the corrosion resistant 
film generation as follows. 
First, heat of working is generated during rolling to cause peel-off or 
adhesion of titanium, whereby unevenness is formed on the titanium 
surface. The oil for rolling becomes entrained in that unevenness or is 
baked to be caught by the titanium. The rolling oil, which is firmly 
caught throughcontact with active titanium or the scab of titanium, is not 
scattered outside by subsequent heat treatment. But by the heat treatment 
at a temperature as same as or higher than the decomposition temperature 
of the oil, titanium, which is a kind of active metal reacts with the 
decomposed oil to form products of Ti(CN), TiC, Ti.sub.2 N, and, by the 
film products, corrosion resistance is remarkably improved. 
From these considerations, it can be understood that the necessary 
conditions for the present invention are the three of (1) presence of oil, 
(2) catching of oil by working and (3) heat treatment. The kind of oil is 
not limited to the oil for rolling, but any oil similar thereto may be 
employed. It has also found by us that catching of oil is influenced 
primarily by the degree of working. 
FIG. 1 shows the result of X-ray diffraction intensity of Ti(CN) and 
corrosion tests of the samples which was taken at appropriate rolling 
reduction, when pickled titanium coil of 0.5 mm thickness (Grade 2) was 
cold-rolled to 0.2 mm thickness with a oil, and subsequently annealed at 
650.degree. C. for 3 hours. X-ray diffraction was performed by the use of 
a Cu tube bulb, under the conditions of a tube current of 16 mA, a tube 
voltage of 30 KV, and the peak at a diffraction angle (2.theta.) of 
36.1.degree. was taken as the diffraction intensity of Ti(CN). 
On the other hand, corrosion resistance was evaluated by the durable time, 
namely how long the corrosion did not start after the sample was dipped 
into a boiled 5% HCl aqueous solution. The start time of the corrosion was 
confirmed by generation of hydrogen gas and weight reduction of the 
sample. Under such conditions, corrosion of ordinary titanium without 
corrosion resistant film according to the present invention begins 
simultaneously with dipping, whereby generation of hydrogen gas and weight 
reduction can be observed. 
As can be seen from FIG. 1, in the sample material before rolling, no 
Ti(CN) is observed at all, and it can be seen that corrosion also 
commences immediately in the corrosion test. The X-ray diffraction 
intensity of Ti(CN) of the cold-rolled sample is substantially increased 
in proportion to its working reduction, and improvement of corrosion 
resistance can be seen substantially correspondingly. However, at a 
working reduction less than 10%, although the intensity of Ti(CN) may be 
elevated, perhaps due to the existing amount of Ti(CN) which is yet small, 
no remarkable increase of corrosion resistance can be seen. From this 
fact, it becomes necessary to regulate the lower limit of the working 
reduction to 10%. 
Furthermore, the factors influencing corrosion resistant film formation of 
Ti(CN), etc., include rolling speed, amount of rolling oil, product 
dimensions, etc. However, these factors will have no vital influence on 
the fluctuations under the conventional conditions for rolling pure 
titanium. For example, the rolling speed of titanium is ordinarily 100 to 
300 m/min., but even when rolling is performed at an extremely slow speed 
of 10 m/min., or, on the contrary, at a high speed of 600 m/min., 
formation of corrosion resistant film such as Ti(CN), etc., was confirmed. 
Also, as to the amount of oil for rolling, rolling is generally performed 
while causing an oil for rolling to flow, but even when rolling is carried 
out only with the oil for rolling adhering to the roll with flow of the 
oil for rolling stopped, corrosion resistant film of Ti(CN), etc., could 
be sufficiently formed. With respect to product dimensions, in both a 
titanium coil of 1 ton and a titanium of only 50-mm width and 300-mm 
length, Ti(CN) was observed. 
While the manner in which oil is entrapped on the titanium has been 
described above, a corrosion-resistant film cannot be obtained only by 
such treatment, but the oil is decomposed by subsequent heat treatment at 
a temperature of 300.degree. C. or higher to produce films of Ti(CN), 
Ti.sub.2 N and TiC. 
Ordinarily, such heat treatment is conducted in vacuum or in an inert gas, 
but the effect of corrosion resistance is not changed even by heat 
treatment in the air, although oxide films of TiO, TiO.sub.2 may be 
formed. The heat treatment temperature is preferably from 550.degree. C. 
to 870.degree. C., and by heat treatment within this range, complete 
decomposition of the oil and the reaction with titanium occur, whereby an 
even better titanium product together with excellent micro-structure can 
be obtained. 
The layer (film) of excellent corrosion resistance of the present invention 
contains generally TiO and other complex oxides. The present invention is 
intended to include also these as a matter of course. 
As the method for practicing the above invention, for example, cold working 
is performed in the presence of the oil, and after 10% or more working 
reduction is operated, heat treatment is carried out at 300.degree. C or 
higher in vacuum or an inert gas (or in the air when the surface may be 
oxidized), whereby a titanium material of remarkably excellent corrosion 
resistance can be simply obtained. 
EXAMPLES 
For presenting evidence of the justification of the constitution of the 
present invention and its mechanism as described above, the following 
examples are set forth. 
A pure titanium (Grade 2) plate with a thickness of 2 mm, cleaned of 
contamination, etc., on the surface by pickling as the sample material, 
was subjected to cold rolling to working degrees of 5%, 10%, 40% and 70%, 
and subjected to no rolling whatsoever (working degree 0%), for two cases 
of using and not using a rolling oil. Subsequently, they were heat-treated 
respectively at from 200.degree. to 1000.degree. C. in vacuum. The 
specimens which was not cold-rolled or heat-treated were also ready as a 
comparison. Furthermore, the specimens which was just painted with an oil 
without cold-rolling and subsequently heat-treated in vacuum were also 
ready Table 1 shows the results of testing the specimens mentioned above. 
In Table 1, evaluation of corrosion resistance was performed by the general 
corrosion test and the crevice corrosion test. Corrosion resistance of the 
whole surface corrosion was measured by dipping the sample material in a 
boiled 5% aqueous HCl solution, and a test piece with weight reduction one 
hour later or 10 hour later, was judged to have incurred general 
corrosion. Corrosion resistance to the crevice corrosion was measured by 
dipping crevice corrosion test pieces (one having a gap formed on the 
titanium surface) in a boiled 10% aqueous NaCl solution and taking out the 
sample after 5 days to examine whether crevice corrosion occurred or not. 
The probability of crevice corrosion was calculated from the tests 
mentioned above. 
As can be seen from Table 1, first for the materials not rolled, it can be 
seen that corrosion resistance cannot be improved at all even when heat 
treatment is carried out after coating of a rolling oil. 
Also, even when cold rolling of 10% or more is carried out (rolling at 
300.degree. C. or lower temperature carried out), no improvement of 
corrosion resistance can be seen as far as oil is not used and/or 
heat-treated at 200.degree. C. or lower temperatures. 
TABLE 1 
______________________________________ 
(Results of corrosion resistance tests of 
various treated materials) 
(Note 2) 
Working Presence Heat treat- 
(Note 1) 
Probability 
reduc- of the oil 
ment tem- General 
of crevice 
tion for roll- 
perature corrosion 
corrosion 
% ing (.degree.C.) 
resistance 
(%) 
______________________________________ 
no heat 
0 Painted treatment X 100 
with an 200 X 100 
oil 300 X 90 
700 X 100 
1000 X 80 
Not no heat X 90 
painted treatment 
with an 200 X 100 
oil 300 X 90 
700 X 90 
1000 X 100 
5 Cold- no heat X 80 
rolled treatment 
with an 200 X 80 
oil 300 X 90 
700 X 100 
1000 X 90 
Cold- no heat X 100 
rolled treatment 
without 200 X 80 
any oil 300 X 100 
700 X 70 
1000 X 100 
10 Cold- no heat X 100 
rolled treatment 
with an 200 X 100 
oil 300 .DELTA. 
40* 
700 .DELTA. 
30* 
1000 .DELTA. 
30* 
Cold- no heat X 100 
rolled treatment 
without 200 X 100 
any oil 300 X 100 
700 X 90 
1000 X 100 
40 Cold- no heat X 70 
rolled treatment 
with an 200 X 90 
oil 300 .circle. 
0* 
700 .circle. 
0* 
1000 .circle. 
0* 
Cold- no heat X 90 
rolled treatment 
without 200 X 100 
any oil 300 X 100 
700 X 100 
1000 X 100 
70 Cold- no heat X 100 
rolled treatment 
with an 200 X 100 
oil 300 .circle. 
0* 
700 .circle. 
0* 
1000 .circle. 
0* 
Cold- no heat X 90 
rolled treatment 
without 200 X 80 
any oil 300 X 100 
700 X 100 
1000 X 100 
______________________________________ 
Note 1: 
.circle. not corroded even after 10 hours 
.DELTA. corrosion occurred within 1 to 10 hours 
X corrosion occurred within 1 hour 
Note 2: 
Probability of crevice corrosion (%) 
=- 
##STR1## 
The mark * indicates the method according to the present invention. 
On the other hand, among the specimen which was cold-rolled to more than 
10% working reduction, the test pieces which was cold-rolled with an oil 
and subsequently heat-treated at more than 300.degree. C., have perfect 
corrosion resistance because of being free from not only general corrosion 
after 5 hours but also crevice corrosion after 5 days from the result of 
Table 1, whereby it can be seen how the material prepared according to the 
process of the present invention has excellent corrosion resistance. 
In order to clarify the mechanism of such remarkable improvement of 
corrosion resistance, the surface of the pure titanium plate prepared 
according to the process of the present invention was subjected to X-ray 
analysis. As a result, a chart as shown in FIG. 2 was obtained. Except for 
peaks those of titanium, those of Ti.sub.2 N, TiC and Ti(CN) were 
observed, so that it could be seen that these corrosion resistant 
materials were formed on the titanium surface. 
On the other hand, the result of X-ray diffraction of the surface of the 
pure titanium plate which was cold-rolled with an oil and subsequently did 
not heat-treated is shown in FIG. 3, in which no peak other than those of 
titanium appears. From these facts, it can be seen that the rolling oil 
adhering firmly during rolling is decomposed by heat treatment to form 
Ti.sub.2 C, TiC, Ti(CN), whereby corrosion resistance is improved. 
The oil used in the tests mentioned above was for rolling, but otherwise, 
oils such as heavy oil, kerosene oil, light oil, lubricant oil, etc., can 
also be used to give similar effects. 
Also, the working reduction of the present invention means the total 
working reduction because the corrosion resistant film of the present 
invention can be continuously formed even when the step of not eliminating 
the titanium surface such as annealing or degreasing is included in the 
process. When the step of eliminating the titanium surface such as 
pickling, polishing, etc., is included in the process, the process of 
forming the corrosion resistant film is interrupted. 
The material according to the present invention is not regulated to only 
pure titanium. It also includes corrosion resistant titanium alloys such 
as Ti-Pd, Ti-Ni-Mo, Ti-Ru-Ni, and Ti-Ta alloys, and construction titanium 
alloys such as Ti-6Al-4V, Ti-15V-3Al-3Sn-3Cr, Ti-5Al-2.5Sn because such 
titanium alloys can easily form Ti(CN), Ti.sub.2 N and/or TiC on their 
surface by working as well as in the case of pure titanium. 
As is apparent from the above example, the titanium material produced 
according to the process of the present invention has remarkably high 
corrosion resistance, and therefore it can be used under an environment of 
aqueous solutions of HCl, H.sub.2 SO.sub.4, HNO.sub.3, etc., in chemical 
plants or places where gap corrosion is likely to occur. Also, it is 
available for batteries. Particularly in the case of using strong 
corrosive substance such as lithium battery, pure titanium (produced not 
according to the present invention) may be sometimes corroded. In this 
case, the titanium material according to the present invention has been 
recognized to be amply resistant under such an environment. 
As an example, when the titanium material according to the present 
invention and other titanium materials were subjected to lath working, 
then coated with carbon fluoride and so on as the active material, and 
resistance was measured after a certain period of time, the material 
according to the present invention was found to have low resistance of 
2.OMEGA., while a titanium material other than that of the present 
invention acquires an extremely high resistance of 7.OMEGA., which is 
unsuitable for a battery. When carbon fluoride was removed and the surface 
was observed by SEM, it was found that corrosion products were formed on 
the surface of the titanium material other than that of the present 
invention. Thus, it was understood that corrosion products were resulted 
from corrosion, whereby resistance was increased. The material according 
to the present invention was found to undergo no change whatsoever on the 
surface without corrosion as the result of SEM observation. 
From these results, the titanium material according to the present 
invention is also the optimum as a material for batteries. 
According to the process of the present invention as described above, since 
a layer containing Ti.sub.2 N, TiC, Ti(CN) is formed on the surface of the 
titanium material, a titanium material of excellent corrosion resistance 
can be provided.