Electrolysis: processes – compositions used therein – and methods – Electrolytic coating – Depositing predominantly alloy coating
Reexamination Certificate
2003-01-10
2004-11-02
Wong, Edna (Department: 1753)
Electrolysis: processes, compositions used therein, and methods
Electrolytic coating
Depositing predominantly alloy coating
C205S311000, C106S001290
Reexamination Certificate
active
06811673
ABSTRACT:
The present invention relates to a process for the electrolytic coating of metals with zinc or a zinc alloy, to an electrolyte composition for the electrolytic coating of steel or iron with zinc or zinc alloys, and to the use of additives for improving the surface roughness and preventing dendritic edge growth in the electrolytic coating of metals with zinc or a zinc alloy.
TECHNICAL FIELD
Zinc coatings offer very good protection against atmospheric influences and are employed for the protection of metals against corrosion. The galvanization of metals, in particular iron or steel, is used on a large scale, for example for the automobile sector. In addition, wires, for example for the electronics industry, belts and tubes are also galvanized on a large scale.
The corresponding workpieces are often zinc plated, since this has advantages over other galvanization processes, such as hot-dip galvanization, sherardization and spraying methods:
a) free choice of the thickness of the zinc layer;
b) no formation of brittle, intermetallic compounds at the iron/zinc interface;
c) low energy requirement;
d) no change in the workpieces due to the effects of heat;
e) no production of hard zinc or zinc ash;
f) clean working, since the galvanization is carried out at relatively low temperatures between room temperature and about 70° C. and no health-damaging vapors are released;
g) more uniform zinc coatings.
Zinc plating can be carried out either in acidic or in alkaline/cyanide electrolytes. Cyanide-based zinc electrolytes give smooth, finely crystalline precipitates. The throwing power of these baths is very good, but the current yield is poor, i.e. electrolysis can only be carried out at relatively low current densities. However, the current density is proportional to the coating rate. It is therefore desirable, for economic reasons, to carry out the electrolysis at the highest possible current densities and thus to obtain the fastest possible zinc deposition.
In the area of continuous strip galvanization, for example for the automobile industry, and the galvanization of wires, strips and tubes, preference is given to acidic electrolytes since fast zinc deposition is possible as a consequence of the ability to use high current densities of up to 200 A/dm
2
at the same time as adequate mobility of the electrolyte and a current yield of virtually 100%. The electrolytes usually used are based on chloride or sulfate.
However, the high current densities result in numerous problems in electrolytic galvanization. Thus, increased edge roughness of the galvanized workpieces is observed owing to dendrite growth and “burn” of the edges. Zinc dendrites broken off during the galvanization or during subsequent treatment of the workpieces also damage the remaining galvanized surface. Furthermore, high current densities cause increased roughness of the entire zinc layer, which may result, inter alia, in problems in applying further layers and also in reduced repulsion of oil or other lubricants used in shaping of the galvanized workpieces, for example in the automobile industry. Finally, grain growth of the zinc layer is difficult to control at high current densities.
In spite of all these disadvantages, high current densities are desirable in electrolytic galvanization owing to the high coating rate.
BACKGROUND ART
Numerous processes which propose possible solutions to the problems which occur in electrolytic galvanization at high current densities are known from the prior art.
U.S. Pat. No. 4,207,150 discloses aqueous cyanide-free electrolytes for electrolytic galvanization which contain a water-soluble zinc salt and in which a quaternary butyl nicotinate salt is employed as brightening and leveling additive. In addition, polyether is preferably additionally employed as brightening agent and methanesulfonic acid and its salts as leveling agent. The advantages of the additives employed can be observed at pH values of from 2 to 7.5.
U.S. Pat. No. 5,616,232 relates to a process for the electrolytic deposition of zinc/chromium alloys in an acidic electrolyte. As additives, use is made of polyethyleneoxyphenol derivatives, which promote deposition of the zinc/chromium alloy.
EP-A 0 727 512 relates to the electrolytic deposition of zinc at high current densities. In this process, an electrolyte is employed which comprises zinc sulfate in an aqueous, acidic electrolysis bath. In this electrolysis bath, the formation of dendrites and edge burn of the workpiece and the roughness of the zinc surface are reduced and the grain size is controlled. As additives, high-molecular-weight polyoxyalkylene glycols are added to the electrolyte as grain size reducers in combination with sulfonated products of the condensation of naphthalene and formaldehyde as antidendritic reagents.
EP-A 0 807 697 relates to electrolytes for electrodeposition of zinc at high current densities and a pH of from 2 to 5 which are said to reduce the usual problems which occur at these current densities. These electrolytes essentially consist of a zinc salt selected from zinc sulfate and/or an organozinc sulfate, and a polyoxyalkylene glycol of low molecular weight based on alkylene oxides having 2 to 4 carbon atoms, an aromatic sulfonate and a conductivity-increasing salt, preferably a potassium salt.
EP-A 0 786 539 likewise relates to electrolytes for electrodeposition of zinc at high current densities which are said to reduce the usual problems which occur at these current densities. Use is made here of an electrolyte based on methanesulfonic acid and a water-soluble organozinc sulfonate. At a pH of >2.5, the use of additional additives is unnecessary, while at a pH of from 1.5 to 2.5, an additive is additionally necessary. This additive is a polyoxyalkylene glycol homopolymer or copolymer based on alkylene oxides having 2 to 4 carbon atoms. Furthermore, the electrolytes in accordance with this application may additionally optionally comprise water-soluble boron oxide compounds, lignin compounds and/or a sulfonated product of the condensation of naphthalene and formaldehyde.
“Zinc-Based Steel Coating Systems: Production and Performance”; edited by F. E. Goodwin, The Minerals, Metals & Materials Society, 1998, pages 293 to 301, describes the advantages of zinc/methanesulfonic acid electrolytes compared with classical zinc sulfate electrolytes. However, fine-grained zinc surfaces are only obtained on use of a grain size reducer which modifies the microstructure and orientation of the deposited zinc surface. No details are given on the grain size reducer employed.
Thus, a suitable electrolyte system for the deposition of zinc or zinc alloys at high current densities (high-speed deposition) which reduces or completely prevents the disadvantages in high-speed deposition in a simple manner in a broad pH range and current density range is still desirable.
It is an object of the present invention to provide a process for the electrolytic deposition of zinc or zinc alloys at high current densities which reduces or prevents the disadvantages which occur in the prior art, such as increased edge roughness of the galvanized workpieces owing to dendrite growth and edge burn, increased roughness of the entire zinc layer and problems in controlling grain growth of the zinc layer.
DISCLOSURE OF THE INVENTION
We have found that this object is achieved by a process for the electrolytic coating of metals with zinc or a zinc alloy in which matt surfaces are obtained, by deposition of zinc from an electrolyte solution comprising a zinc salt selected from zinc sulfate or an alkanesulfonate of zinc or mixtures thereof, and, if desired, further metal salts, an acid selected from sulfuric acid or an alkanesulfonic acid or a mixture of the two acids, and at least one additive for improving the surface roughness and preventing dendritic edge growth, selected from nitrogen-containing surface-active compounds, which may be ionic or nonionic, sulfur-containing anionic surface-active compounds, and surface-active compounds based on multifunctional alcohols having at leas
Brodt Gregor
Haas Jens
Hesse Werner
Jäger Hans-Ulrich
BASF - Aktiengesellschaft
Keil & Weinkauf
Wong Edna
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