U.S. patent application number 12/063338 was filed with the patent office on 2009-08-13 for electropolishing method.
This patent application is currently assigned to POLIGRAT GMBH. Invention is credited to Olaf Bohme, Siegfried Piesslinger-Schweiger.
Application Number | 20090200178 12/063338 |
Document ID | / |
Family ID | 36973892 |
Filed Date | 2009-08-13 |
United States Patent
Application |
20090200178 |
Kind Code |
A1 |
Piesslinger-Schweiger; Siegfried ;
et al. |
August 13, 2009 |
ELECTROPOLISHING METHOD
Abstract
The present invention describes a cost-saving and
environmentally conserving process for the electrochemical
polishing of steel, in particular of low-alloy steels. The
workpieces are rinsed after electropolishing in an arid phosphoric
acid-sulfuric acid-bath in a first rinsing step with a phosphoric
acid-containing solution, whereby a chemical attack on the freshly
polished surfaces is avoided without the use of environmentally
harmful and unhealthy inhibitors such as chromates. By recycling of
the acids and the rinsing solution, these acids and solutions can
be regenerated, whereby the process can be designed
effluent-free.
Inventors: |
Piesslinger-Schweiger;
Siegfried; (Vaterstetten, DE) ; Bohme; Olaf;
(Erding, DE) |
Correspondence
Address: |
WESTERMAN, HATTORI, DANIELS & ADRIAN, LLP
1250 CONNECTICUT AVENUE, NW, SUITE 700
WASHINGTON
DC
20036
US
|
Assignee: |
POLIGRAT GMBH
Munchen
DE
|
Family ID: |
36973892 |
Appl. No.: |
12/063338 |
Filed: |
July 31, 2006 |
PCT Filed: |
July 31, 2006 |
PCT NO: |
PCT/EP06/07583 |
371 Date: |
March 20, 2009 |
Current U.S.
Class: |
205/680 ;
205/682 |
Current CPC
Class: |
C25F 7/02 20130101; C25F
3/24 20130101 |
Class at
Publication: |
205/680 ;
205/682 |
International
Class: |
C25F 3/24 20060101
C25F003/24 |
Foreign Application Data
Date |
Code |
Application Number |
Aug 9, 2005 |
DE |
10 2005 037 563.4 |
Claims
1. A method of electrochemical polishing of low-alloy steels using
an electrolyte containing 100 to 30 wt. % phosphoric acid and 0 to
70 wt. % sulfuric acid and in which the electrolyte is rinsed off,
characterized in that a solution containing phosphoric acid is used
for the rinsing, where the phosphoric acid content of the rinsing
solution is at least 50 wt. %.
2. The method as claimed in claim 1, characterized in that
concentrated phosphoric acid is used for the rinsing.
3. The method as claimed in claim 1 or 2, characterized in that the
electrolyte is substantially chromium-free.
4. The method as claimed in claim 1 or 2 characterized in that the
electrolyte has a water content of max. 20 wt. %.
5. The method as claimed in claim 1 or 2, characterized in that the
electrolyte contains 80-50 wt. % phosphoric acid and 20-50 wt. %
sulfuric acid.
6. The method as claimed in claim 1 or 2, characterized in that the
electrolyte contains iron ions in an amount that inhibits chemical
attack on the surface of the steel.
7. The method as claimed in claim 6, characterized in that the
electrolyte contains iron ions in an amount of at least 1 wt.
%.
8. The method as claimed in claim 6, characterized in that the
electrolyte has more than 2.0 wt. %, though at most approx. 8% iron
ions.
9. The method as claimed in claim 1 or 2, characterized in that the
electrochemically polished steel is rinsed with water following the
rinsing with solution containing phosphoric acid.
10. The method as claimed in claim 9, characterized in that the
inorganic acids contained in the rinsing water are recovered.
11. The method as claimed in claim 1 or 2, characterized in that
the electrolyte is supplemented at least partially with the
electrolyte-enriched phosphoric acid from the first rinsing stage
and optionally with phosphoric acid recovered from at least one
other water-based rinsing stage.
12. The method as claimed in claim 1 or 2, characterized in that
the electrolyte is supplemented at least partially with the
electrolyte-enriched phosphoric acid from the first rinsing stage
and optionally with phosphoric acid recovered from at least one
other water-based rinsing stage.
Description
[0001] The present invention describes a method for the
electrochemical polishing of steel workpieces, in which corrosive
attack of the surface in the rinsing operation can be avoided even
without the use of expensive and environmentally harmful
inhibitors. Said method is also suitable in particular for
workpieces of low-alloy steels, which are especially susceptible to
chemical attack.
[0002] Electrochemical polishing is a process that is used for the
deburring, smoothing and brightening of metal surfaces. Owing to
the higher current density at fine scratches and other
irregularities, the metal is ionized and dissolved at these points
faster than on the smooth regions of a metal workpiece, so that its
irregularities are leveled out. For this purpose, the objects that
are to be electropolished, which are suspended on suitable carriers
or are arranged in baskets or the like, are immersed in an
electrolyte, the polishing bath, and are removed from the latter
after a certain time. After the bath liquid has drained from the
polished surfaces, the objects are immersed in rinsing baths, to
remove the electrolyte completely. These electropolishing methods
have found extensive industrial application, in particular for
steels with a chromium content above 12%, which are generally
called stainless steels, special steels or acid-resistant steels.
The electrolytes used are mainly based on mixtures of phosphoric
acid and sulfuric acid, and brighteners and inhibitors can be added
to them for further improvement of the action.
[0003] However, steels with a chromium content below 12%, i.e.
low-alloy steels such as structural steels and tool steels, which
form the majority of the steel grades used, cannot be
electropolished with adequate quality with the methods used for the
processing of special steels. The reason for this is the lower acid
resistance of these steels, which leads to uncontrollable chemical
and corrosive attack on the surfaces by the electrolyte during the
electropolishing process.
[0004] For successful electropolishing of low-alloy steels with
electrolytes based on phosphoric acid and sulfuric acid, generally
a notable concentration of chromic acid, i.e. an oxyacid of
hexavalent chromium (chromate), is added as inhibitor to the
electrolytes, and prevents chemical attack on the surfaces of the
workpiece during electropolishing.
[0005] Chromates are highly toxic, embryotoxic and carcinogenic, so
their industrial use is increasingly being restricted and is
subject to stringent safety requirements regarding labor protection
and environmental protection. Patent JP-A 5 163 600 describes the
addition of ascorbic acid or salts of ascorbic acid as a possible
means of reducing hexavalent chromate to chromium-(III) ions, which
are less toxic. However, the use of chromic acid also constitutes
an appreciable cost factor, which further limits the economic
effectiveness of the electropolishing process.
[0006] Various steels, aluminum, nickel and alloys thereof are
electropolished, according to U.S. Pat. No. 2,773,821, in solutions
of sulfuric and phosphoric acid without chromic acid, although
addition of hydroxyacetic acid, benzenesulfonic acid and
toluenesulfonic acid is necessary. The organic additives account
for up to 40% of the electropolishing solution. EP-A 0 249 650
uses, for the chromium-free electrochemical polishing of objects of
steel, special steel, nickel alloys, aluminum and aluminum alloys,
a chelating agent based on phosphonic acid. EP-A 1 443 129
describes the addition of up to 50% of alcohols and other
surface-active substances to the electropolishing solution, before
the electropolished objects are varnished. All these additives
represent a not inconsiderable cost factor.
[0007] The rinsing process that follows the electropolishing
operation, and is intended to remove adhering electrolyte from the
surfaces, is of decisive importance for the brightness and
smoothness of the object that is processed. The resultant decrease
in the concentration of the acid on the surface of the
electropolished object increases the corrosive action of the
electrolyte. This effect should be suppressed by the addition of
inhibitors such as chromic acid. Without these additives, the
freshly polished metal surfaces are attacked again, so that to a
considerable extent the effect of smoother and brighter surfaces
achieved by the electropolishing is lost.
[0008] Therefore an electropolishing method for low-alloy steels,
in which this chemical attack by the dilute acid can be avoided
even without the addition of expensive substances that are also
harmful to the environment and to health, and is comparable to the
methods for the processing of special steels with respect to costs
and potential risks, would be of considerable advantage to
industry.
[0009] DE 808 519 B describes a method for the polishing and
deburring of high-carbon or low-carbon steels and low-alloy steels
by electrolytic means. The electrolyte contains 5 to 60 wt. %
sulfuric acid and 30 to 80 wt. % phosphoric acid. The electrolyte
bath can additionally contain a trivalent dissolved metal, e.g.
iron, among others.
[0010] AT 190 769 B describes a method and an electrolyte for the
electrolytic cleaning of metal objects. The electrolyte consists of
hydrochloric acid and is rinsed following electrolysis, and
phosphoric acid in the proportion of about 0.05% to 3% can be added
to the rinsing water. The purpose of adding phosphoric acid is to
prevent the formation of oxides on the metal of the treated
object.
DETAILED DESCRIPTION OF THE INVENTION
[0011] The invention presented here is based on an electropolishing
method, which, like the electropolishing method for special steel,
is based on mixtures of phosphoric acid and sulfuric acid, in which
the first rinsing step following the actual electropolishing step
is performed with a solution containing phosphoric acid, preferably
with a solution that has a phosphoric acid content of at least 50
wt. %. In particular, the use of concentrated phosphoric acid, with
a content of 85 wt. % H.sub.3PO.sub.4, is suitable here as the
starting solution. This method manages without the addition of
chromic acid or other inhibitors and therefore offers considerable,
and not only economic, advantages.
[0012] First the objects that are to be electropolished are
degreased in an optional step, to avoid contaminating the
electrolyte and to make the surfaces of the workpieces completely
accessible for the electrolyte. Any commercial degreasing solution
can be used for this. Next, the workpieces are usually rinsed with
water and then immersed in the electropolishing bath and connected
as the anode. Undesirably severe chemical attack on the surfaces of
the object being electropolished can be prevented during the
electropolishing step by keeping the water content of the
electrolytes low. Therefore highly concentrated acids, such as
sulfuric acid, phosphoric acid and mixtures of sulfuric acid and
phosphoric acid are used almost exclusively for the
electropolishing of steels and steel alloys. Electrolytes with a
water content of max. 20 wt. % give particularly good effects.
[0013] Moreover, it proves advantageous if, right at the beginning,
the electrolyte already contains iron-(III) ions at a concentration
of at least 1 wt. %, preferably at a concentration above 2.0 wt. %.
In order to achieve chemical activity that is sufficient for an
economical process, the temperature of the electrolyte should be
above 50.degree. C., preferably 60.degree. C. to 90.degree. C.
[0014] The problem of chemical attack during decrease of the
concentration of acid in the course of the rinsing process, without
the use of inhibitors, was solved in the method according to the
invention by rinsing at room temperature with concentrated
phosphoric acid with low water content, rather than with water, in
the first stage of the rinsing process. Surprisingly, it is found
that, following this first rinsing step, the surfaces can then be
finish-rinsed with water without any problem, without chemical
attack by the dilute acid being observed. It is advantageous to add
to the rinsing water, in the last rinsing step, a certain
proportion of a commercial corrosion inhibitor such as KORANTIN BH
(2-butyne-1,4-diol), to prevent subsequent corrosion during
drying.
[0015] It is found that enrichment of the phosphoric acid with
electrolyte in the first rinsing stage up to a sulfuric acid
content of approx. 20 wt. % does not adversely affect the results.
This offers the possibility of using the phosphoric acid, enriched
with sulfuric acid electrolyte, in its turn as the basis for the
production of fresh electrolyte. Recovery of the phosphoric acid
entrained in the additional rinsing process from the rinsing water
is possible without loss of quality. This makes recovery of the
inorganic acids, in conjunction with circulating the rinsing water
through an evaporator, extremely economical. In this way the
electropolishing process can be designed to be almost
effluent-free.
[0016] The iron ions abstracted from the workpiece surface during
electropolishing go into solution in the electrolyte and accumulate
there. Beyond a critical concentration of approx. 8 wt. %,
equivalent to approx. 140 grams of iron per liter in the
electrolyte, there is a marked decrease in efficacy of the
electrolyte. This necessitates a decrease in the iron content by
partial exchange with fresh electrolyte. The used electrolyte can
either be removed directly, or by drag-out to the rinsing
process.
[0017] After removal, the spent electrolyte should either be
delivered to a licensed site for destruction, or should be made
usable again by regeneration. Electrolytic precipitation of the
iron in the form of Fe(II) sulfate from the concentrated
electrolyte is eminently suitable for regeneration of the spent
electrolyte. Thus, finally the iron removed in the form of iron(II)
sulfate is the only waste product from the electropolishing
process, and for its part it may find further industrial use,
perhaps as a reducing agent.
[0018] Using the method according to the invention it is therefore
also possible for low-alloy steels to be electropolished just as
efficiently and inexpensively as special steel. Moreover, this
process also represents a method of electropolishing that is far
less harmful to the environment and presents less risk to
health.
[0019] The invention is explained in more detail in the following
examples. The examples only represent possible embodiments of the
electropolishing method described here, and in no way imply a
restriction to the conditions presented here.
EXAMPLES
Example 1
[0020] A set of cutting tools made of hardened tool steel (material
No. 1.3343) was electropolished in an electrolyte consisting of 50
wt. % phosphoric acid and 50 wt. % sulfuric acid with a specific
gravity of 1.75 kg/l and an iron content of 4.5 wt. % at an
electrolyte temperature of 80.degree. C., current density of 40
A/dm.sup.2 and voltage of 12 V for 6 min, and then prerinsed in
concentrated phosphoric acid (85 wt. %) at room temperature, rinsed
finally in water, then immersed in water at a temperature of
60.degree. C., to which a commercial corrosion inhibitor was added
at a concentration of 2 wt. %, and dried in air.
[0021] A second set was electropolished in an electrolyte with 70
wt. % phosphoric acid, 2.5 wt. % sulfuric acid and 9 wt. % chromic
acid with a specific gravity of 1.740 kg/l and an iron content of
2.5 wt. % at an electrolyte temperature of 50.degree. C., current
density of 40 A/dm.sup.2 and voltage of 11 V for 6 min. The parts
were then rinsed with water and dried.
[0022] The result of electropolishing was the same in both methods
with respect to leveling of the surfaces and smoothing of the cut
edges.
Example 2
[0023] Plates of heat-treatable steel, in the hardened and
unhardened state, were electropolished in electrolytes according to
Example 1. The current density was 25 A/dm.sup.2 at 14 V and
electropolishing time of 60 min. The rinsing process was carried
out as described in Example 1, as well as drying in air. The
results achieved for the hardened and the unhardened plates were
the same in both methods with respect to material removal,
brightness and leveling.
* * * * *