Electrolysis: processes – compositions used therein – and methods – Electrolytic coating – Depositing predominantly single metal or alloy coating on...
Reexamination Certificate
2000-06-14
2002-04-02
Gorgos, Kathryn (Department: 1741)
Electrolysis: processes, compositions used therein, and methods
Electrolytic coating
Depositing predominantly single metal or alloy coating on...
C205S103000, C205S106000, C205S323000, C205S326000
Reexamination Certificate
active
06365028
ABSTRACT:
AREA OF TECHNOLOGY
This invention relates to processes for applying protective oxide coatings to items made from aluminium alloys, and more specifically, to a method of plasma electrolytic oxide coating of the surfaces of items. The invention may be used in engineering, equipment-building and other areas of industry.
Because of their physical and mechanical properties and the process used to manufacture items of complex configuration, aluminium alloys (both wrought and castable) are increasingly being used in the manufacture of important and rapidly-wearing parts of machines. There is therefore an urgent need for protective coatings to be produced thereon which are resistant to wear when exposed to abrasive particles and high local temperatures, and are unaffected by corrosive environments. One way of dealing with this problem is to apply ceramic-oxide corundum coatings to aluminium alloys using a method of plasma electrolytic oxide coating. Of crucial importance for long-term operation of items with such a coating is the thickness, micro-hardness and strength of adhesion to the substrate of the coating, while for the method to be assimilated in practice, the process needs to have a high output and be reliable, the equipment should be simple and the way it is run should present no hazard to the environment.
PRIOR ART
A method is known for oxidising aluminium alloys (DE, A1, 4209733) in an anode-cathode regimen with a current density of 2-20 A/dm
2
and final voltage amplitudes of: anode—300-750 V; cathode—15-350 V. The pulse frequency may vary from 10 to 150 Hz, with the anode current pulse duration 10-15 ms and the cathode current pulse duration 5 ms. The method enables dense solid oxide coatings 50-250 microns thick to be applied using an alkaline-silicate or alkaline-aluminate electrolyte.
This method has the following drawbacks: low process output, high energy consumption and complex equipment requirement In addition, use of the traditional alkaline-silicate electrolyte does not ensure that a consistent quality coating is produced on the items. Long-term use of the electrolyte leads to changes in the characteristics of the coatings applied, with a deterioration in the quality and a diminution in the thickness thereof. Electrolyte stability lies within 30-90 Ah/l, and is not capable of being adjusted during the operating process.
A method of obtaining solid, ceramic-oxide coatings of low porosity and with good adhesion to the substrate, 100 microns or more in thickness, on aluminium alloys is known (U.S. Pat. No. 5,616,229). Shaping of the layer takes place in an anode-cathode regimen in sequence in several baths containing an alkaline-silicate electrolyte. Of these baths, the first contains only a 0.5 g/l aqueous KOH solution; the second contains an aqueous solution of 0.5 g/l KOH and 4 g/l sodium tetrasilicate; and the third contains an aqueous solution of 0.5 g/l KOH and 11 g/l sodium tetrasilicate. The main drawback to this known method is the use of a traditional unstable electrolyte, coupled with the complex equipment design and apparatus layout.
Another method is known by which wear-resistant ceramic-oxide coatings may be applied to aluminium alloys (U.S. Pat. No. 5,385,662), 50-150 microns in thickness, using plasma-chemical anode oxide coating with a current density of over 5 A/dm
2
and at an electrolyte temperature of up to 15° C. A very narrow temperature fluctuation range of ±2° C. is allowed. The electrolyte consists of an aqueous solution of sodium phosphate and borate, and also contains ammonium fluoride; the total salts concentration in the solution should not exceed 2 M/l. Use of this electrolyte does not enable a coating with a high micro-hardness rating to be obtained on aluminium alloys (no more than 7.5 GPa). This is also indicated by the low value of the final anode voltage (just 250 V). The electrolyte also contains harmful fluorides, which necessitates expenditure to dispose of these. To obtain coatings with a high level of hardness (up to 20 GPa), the electrolyte described above may, it is proposed, be diluted by 100 times with water and 0.1 M sodium aluminate and 0.1 M sodium silicate added (the pH of such a solution is 10-12). Again, the main drawback to this method is the lack of stability of the aluminosilicate electrolyte. Sodium aluminate is also poorly soluble in water, which gives rise to an oxide coating that is uneven over the thickness of the coating, and to the formation of deposits on the walls of the stainless steel bath that are difficult to remove.
A method is known for applying solid corrosion-resistant coatings to items made of aluminium and its alloys (U.S. Pat. No. 5,275,713) in an aqueous electrolyte solution containing an alkaline metal silicate, hydrogen peroxide and small quantities of hydrogen fluoride, alkaline metal hydroxide and a metal oxide (for example, molybdenum oxide). The solution has a pH of 11.2-11.8. A positive potential is delivered to the item from a direct or pulsed current source. For the first 1-60 s the voltage is raised to 240-260 V, and over the next 1-20 minutes (depending on the required coating thickness) it is steadily increased to 380-420 V. The introduction of hydrogen peroxide as an oxygen accumulator into the electrolyte helps to raise the rate of increase of the oxide coating and its hardness through intensification of oxide coating of the metal in the spark discharge zone.
A drawback to this method, however, is the fluorides and heavy metal salts content in the electrolyte. The heavy metal salts also have a harmful impact on the stability and duration of use of the electrolyte, since heavy metal ions are catalysts and significantly accelerate the breakdown of hydrogen peroxide in solution. Moreover, the “voltage surge” achieved in the first few seconds of the process, while enabling the pre-spark oxide coating period to be somewhat curtailed, has virtually no impact on the properties of the coating, since it is done at relatively low current densities (not above 15 A/dm
2
). This method is used to apply tin oxide films (up to 30 microns) which always have good adhesion to the substrate.
The method that is most similar to the proposed invention is one in which solid ceramic-oxide coatings are applied to items made of aluminum alloys by plasma electrolytic oxide coating (RU, C1, 2070622) in a pulse anode and/or anode-cathode regimen using commercial-frequency current An environmentally clean electrolyte is used, comprising an aqueous solution of an alkaline metal hydroxide, a silicate and an alkaline metal pyrophosphate. The P
2
O
7
−4
pyrophosphate ions stabilise the colloidal silicate solution, and play an active part both in the plasmochemical synthesis of oxides in the spark breakdown channels, and in the processes of electrochemical polycondensation of anion complexes of the electrolyte on the spark-free surface. The electrolyte features a high level of stability (up to 400 A·h/l) and the capacity to be adjusted while in use. A drawback of the known method is the relatively low rate of formation of the oxide coating and the high level of energy consumption of the process.
DISCLOSURE OF SUBSTANCE OF INVENTION
The main aim of this invention is to improve the quality of the ceramic-oxide coating through an increase in the strength of adhesion to the substrate and in the micro-hardness of the coating. Another aim of the invention is to increase the rate of formation of the oxide coating through intensification of the plasmochemical synthesis reactions without increasing the energy consumption of the process. A further aim of the process is to ensure that quality oxide coatings are obtained over a relatively lengthy period of time through use of an electrolyte with a high level of stability and the capacity to be adjusted during use. Yet another aim of the invention is to reduce the cost of running the oxide coating process through the use of simple and reliable equipment with the minimum essential apparatus layout and an environmentally clean electrolyte comprising inexpensive an
Gorgos Kathryn
Isle Coat Limited
Nicolas Wesley A.
Sheridan & Ross P.C.
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