Technical sciences / Processing of materials in engineering

Doctor of Technical Sciences, Professor Boldyrev A.I.,

Candidate of Technical Sciences Boldyrev A.A.

Voronezh State Technical University, Russia

Protection of Industrial Equipment Elements from Anodic Attack during Electro-Chemical Machining

Electrochemical dimensional processing causes electoral dissolution of an anode, which one, as a rule, is a part. However in a number of cases operating current goes through the equipment to the part and it becomes an anode, i.e. It is subjected to dissolution just like a part. This limits the field of application of electrolytic machining, since it results in additional investments for repair and replacement of separate parts and technological equipment as a whole.

In the process of electrolytic machining the basic reasons for destruction of appliances are the redox processes which go on their surfaces under effect of working solutions of electrolytes, atmospheric components and electric current.

The main ways of technological equipment protection are indicated in fig. 1

Ways of technological equipment protection in the process of elecrolytic machining
Design of equipment With improved Protection	Selecting the working environment and Changing its properties	Methods of protection choosing the right conditions	By the meterials That are used	Templates and elements of construction	Coatings

Fig. 1. The main ways of technological equipment protection

from destruction in the process of electrolytic machining

One of the ways of technological equipment protection is perfecting designs ensuring heightened protection. It is errective even at the stage of the design solutions when constructing the working section of the tool. The example of this is creation of great differences of spacing intervals between the part and the working and nonworking surfaces of the tool with the border conterminous to the beginning and the end of machining zone. In this case we design the tool with a fissile segment, maximum approximated to the part, and with a lateral area removed from the part. Such approach helps to solve the problem of process localization in case of dimensionless processing or in the case of dimensional processing with indistinctly expressed borders of the profile, for example, if aftertreatments of transient segments is necessary.

The electrolytes used in the process of elecrtolytical machining become more active when electrolytic current is imposed. Therefore electrical conductivity and speed of dissolution of the equipment metal depend on the structure of the current. For obtaining high technological parameters of the electrolytic machining process the electrolyte should meet the following requirements:

- the spurious reactings lowering a current efficiency should not take place in it even in minimum quanyity;

- the dissolution of a bar should take place only in the zone of processing;

- the operating current should flow past on all segments of the job surface.

For obtaining high technological parameters we try to control the change of electrolyte characteristics, saving its specific conductivity constant on all segments of the job surface during the whole process. The structure of the working environment also influences the degree of electrolytical field localization, so we quite often add to the working environment some actuating components permitting to blast passivating films in the zone of processing. Then the gradient of field intensity on border of the tool can increase and together with other limitations (reduction of backlashes, coatings on the tool etc.) Can become reasonable for application.

The protection of technological equipment parts is possible by controling the process of electrolytic machining in the gap. Achieving the high intensity of pick up in a working area with legible borders makes possible the localization of the process. This principle is used in the case, when in the zone of processing in the gap the grid is set (principle of an electron tube), on which the potential moves, this can speed up the process of dissolution in the grid. The cathodic protection of the adaptation parts of the machine tool is one more regime method. It is implemented by means of apposition of the protector from metal with a more negative equilibrium potential to the protected construction and also by connection of the construction with the negative pole of the separate source of direct current or with the use of the installation of additional electrode - protector connected to the separate power source.

For a technological equipment it is necessary to select corrosion-resistent alloys with high electrical conductivity (in case of voltage will go through technological equipment parts), good wear hardness to local destruction at short circuits, high adhesion to dielectric coatings, sufficient mechanical strength, workability, and low cost. Cuprum, copper alloys, stainless steels meet these requirements best of all; to a lesser degree - titanium alloys (because of low workability). There are different means of protection from corrosion in solutions of electrolytes. For example, for this purpose we can apply rustproof doped metals (or increase of equipment stability with the help of deposition of protective coatings). Other methods are gap of arising electric networks at the expense of the introducing in these circuits of large resistance (for example, by means of inhibitors), choice of contacting metallical parts with close values of electrolytic potentials. For design of appliances in some cases it is possible to pick up metal, with demanded corrosion stability outgoing from value of its electrochemical potential in considered conditions. In the majority of aqueous solutions of electrolyte alloys on the basis of a chromium, niobium, zirconium, molybdenum of a tungsten also have corrosion stability. However the disadvantages of alloys on the basis of these metals are their inferior machining property and high price.

For technological equipment protection we apply constructional parts from dielectrics. They are used for adapters, fastening parts, guides for welding rods of tools working on the scheme of an allowance, horns and other parts. They should be strong and capable to work at heats (from above 320° К), and to have low water occluding ability. Rubber, glass-termazote, fluroplastic and caprolon correspond to these requirements. The most perspective method that helps to achieve this aim is the installation of parts with legiblly restricted contour kept during all the time of processing and having good adhesion with a part. It is accepted to call such elements templates. They can be of many types. Their designs are various. The templates on the tool serve many times, therefore their application justifies costs of composite and reliable designs. At the same time they have limited accuracy.

Today the most effective equipment protection is usage of coatings received by different methods. In industry we apply the following ways of polymers’ spraying: gas-flame, vortical, in an electrical field, electrovortical; less widespread ways are ink-jet, plasma and some others. The requirements to plotted coatings are various. The coatings should have a good adhesion to metals, mechanical strength, good wear resistance in conditions of friction, stable dielectric properties. The coatings should create a dense pore-free film eliminating infiltration of an electrolyte to metal, not inflate under effect of a liquid. As coatings we use epoxies, lacquers, ceramic enamels, capron in a dust, polyethylene and other stuffs. Some sorts of coatings are shown in tab. 1.

Table 1

Structure of coating	The field of application of welding rods with this coating	The depth of coating, mm
Ceramic enamels	Weaving of foramens, narrow slots, shaped cavities	0,03 - 0,08
Polypropylene Epoxy resin /// Nonacryl	Deburring, processing of contours	0,3 – 0,35 0,1 – 0,3 0,2 – 0,5
Fluoroplastic emulsions Polyvinyl Chloride	Stitching foramens of small diameters	0,02 - 0,05 0,08 - 0,2
Organic glass, ebnite, rigid-vinyl, glass textolites Polyurethane resin PU-1	Deburring, processing of large cavities, foramens of large diameters	0,5 – 5 0,1 - 0,2

The usage of oxide layer of metal on the equipment obtained directly on a technological part by its special heat treatment as an insulation blanket is very effective. On the surface of the workpiece appears pore-free glassy coating with the depth 0,1-0,2мм, strongly bound with the base metal and a melting at high temperatures. The proximity of coefficients of base metal linear expansion and isolation has a favourable effect on the strength of isolation at the rise of temperature on the electrolyte. For technological usage of the welding rod - tool of working surfaces and the current leads are protected from metallical luster.

Above-stated facts allow to work out the guidelines on the design of technological equipment that has protection against destruction under a technological current, and also on the structure of handling conditions for obtaining a reliable protective coating.