GB2119298A - Method of abrading surfaces - Google Patents

Abstract

Particulate urea is used as the abrasive in dry abrasive blasting processes.

Description

SPECIFICATION Method of abrading surfaces It is well known to abrade surfaces by dry abrasive blasting methods. Such methods comprise blasting abrasive particles against a surface by directing onto the surface a stream of the abrasive particles entrained in a large volume of gas. Typical abrasive particles that are used include alumina, sand, crushed shells, silicon carbide, powdered glass and metal particles.

If the surfaces are of complex shape there may be a tendency for abrasive particles to be entrapped by the surfaces and to remain on them after the abrasion is completed. This is very undesirable, especially when the surfaces are to serve as components in assemblies that involve moving parts.

During use the particles may become dislodged from the surfaces where they are entrapped and cause severe wear. Examples of such surfaces are pistons, crankshafts and bearings in motor assemblies and hollow blades and other components of jet engines.

It might be thought possible to minimise this problem by using water soluble abrasive particles and water washing the surfaces after the blasting.

Unfortunately none of the water washable particles that have been proposed previously have proved to be satisfactory in practice.

In the invention the abrasive particles are urea particles. If desired the surface that has been blasted with them may be washed with water to remove any that have been entrapped by the surface.

The urea particles generally have a size of at least 0.01 mm since although smaller particles can be used as abrasive there is generally no advantage in using them and there can be the disadvantage of undesirable dusting problems. Normally the particle size is at least 0.05 mm and most preferably at least 0.1 mm. The particles are not usually larger than about 2 or 3 mm in particle size since particles as large as this or larger than this tend to give rather coarse abrasion. However for some purposes this may be desired. Generally the particle size is not more than about 1 mm with values of around 0.1 to 0.6 mm generally being particularly suitable. The particles may be commercial grade crystalline urea but preferably are urea prills.Many commercial grades of urea prills can have a particle size larger than is desired in the invention and so the preferred products are those known as urea microprills, for instance the product supplied by Chemie Linz AG under the name "Biuron" microprilled urea.

The urea particles are preferably used as the sole abrasive in the dry blasting method. They may be directed onto the surface in conventional manner.

Thus they are made to impinge at high velocity onto the surface by entrainment in a high velocity stream of a large volume of air or other inert gas. This stream may be generated by forcing compressed air out of a nozzle. The particles are entrained in the stream, generally before ejection from the nozzle, by conventional means such as a suitably designed hopper or by venturi action. Typically the air pressure is at least 1 kg/cm2 but is generally not more than about 8 kg/cm2. Air pressures of between 2.5 and 6 kg/cm2 are often preferred. The volume of air (measured at standard pressure) is generally very large compared to the volume of urea entrained in it, typically being more than 1,000 times the volume of urea.

The dry blasting is preferably conducted while the surfaces are enclosed within a dry blasting cabinet, but this is not essential. Generally the particles that have been blasted onto the surfaces are collected and reused until they become too contaminated.

After the dry blasting operation has been completed the surfaces may be washed with water, typically at ambient temperatures, for instance by immersion or, more usually, by application of spray or jets of water.

The use of urea results in satisfactory abrasion and avoids the disadvantages of using water insoluble particles or other water soluble particles. Typical water soluble particles that might be considered as alternatives for urea include salt, sugars, sodium benzoate, citric acid and certain other crystalline acids, and salts of various organic and inorganic acids, for instance ammonium bifluoride. However all these other materials have serious disadvantages that are not possessed by urea.

Many of the materials are highly acidic or highly alkaline when dissolved in water and therefore tend to cause corrosion of the surfaces and/or effluent discharge problems. An example is citric acid. Many of the materials accelerate corrosion of metal surfaces even though they may not be acidic or alkaline when dissolved in water. For instance metal surfaces of aluminium, magnesium, steel, titanium, nickel and cobalt based alloys may tend to be corroded when sodium chloride is used as the abrasive and is subsequently washed. Further it is well known that the specialised metals and metal alloys that are often used in components of, for instance, turbine engines and other devices should not be brought into contact with certain elements which many water soluble materials do contain.Thus compounds containing elements such as sulphur, chlorine, fluorine, bromine, iodine, sodium, lead, antimony, bismuth and zinc should not be brought into contact with such components. These criteria alone render unsuitable many water soluble abrasive particulate materials.

Another problem that arises with many water soluble materials is the risk of explosion. Materials such as sugar can result in the formation of inflammable residues or explosive dust clouds during dry blasting where oxygen from the compressed air stream or from the surroundings may admix with finely dispersed powder particles.

Another problem arises from effluent disposal in that many water soluble materials create a solution that is toxic by ingestion and/or is harmful to the environment. Examples are sodium hydroxide, sodium cyanide and oxalic acid.

Finally, many water soluble materials which in theory might be suitable in practice are not commercially available in a particulate form having the desired particle size or powder rheology. Examples include some grades of sugar and sodium laurate.

Urea avoids all these disadvantages, is very water soluble, and is readily available in suitable, free flowing, particulate form. It is non combustible. The commerically available grades of urea generally contain negligible quantities of harmful materials.

Aqueous solutions of urea are only mildly alkaline and so are relatively non-corrosive to sensitive metals. Its existing agricultural and industrial uses have established that it is basically not harmful to the environment. It occurs naturally in the blood stream in measurable concentrations and there is no evidence that it presents any health hazard.

Bythe invention it is therefore possible to abrade simple or complex metallic, or sometimes nonmetallic, surfaces and effectively remove mill scale, corrosion products, unwanted paint and many other kinds of surface contamination. In addition to or instead of this abrasion the blasting with urea can impart a certain roughness to the surface and this may be desirable as a pre-treatment for subsequent surface treatments, for instance to assist adhesion of a subsequentiy applied coating obtained by painting or plating.

The invention is of particular value for the removal of paint, especially after treatment of the paint with a suitable paint remover. For instance a surface carrying a paint coating, or residues of a paint coating, may be immersed or otherwise treated with a paint remover and the partially degraded paint may then be removed by blasting with urea.

The following are examples of the invention, example 1 being comparative.

Example 1 An experiment was carried out using a portable dry blasting cleaning rig of the type marketed by Hodge-Clemco Ltd. In this experiment sugar granules were used as the dry abrasive blast medium and they were fed into a compressed air stream via a powder hopper and ejected with the compressed air at a 6.2 mm nozzle. The air pressure was 40 psi (2.8 kg/cm2).

The jet of particles was made to impinge on a titanium compressor blade from a commercial jet engine, the blade having been previously soiled by running in the engine and covered with a layer of carbonaceous 'sooty' deposits requiring removal before reassembly of the engine and for inspection purposes.

The attempt resulted in a poor cleaning performance in which sugar granules which had broken down on impact adhered to the surface to be cleaned and built-up on that surface preventing any further abrasive cleaning effect on the underlying carbonaceous contamination.

Large volumes of sugar dust were generated in the cleaning attempt and a hazard of combustion and possibly explosion of the dust cloud was present.

Example 2 In this example a further dirty section of the same compressor blade was abraded in a similar cleaning rig but using a different air nozzle and a different abrasive. The air pressure was 80 psi (5.6 kg/cm2) and the ejector nozzle orifice was 3.2 mm. The abrasive particles were of crystalline urea, and in particular the microprilled urea sold under the trade name "Biuron", typically having a particle size of around 0.5 mm.

Blasting was conducted for about 45 seconds, during which about 0.5 kg urea was blasted onto the surface.

A good abrasive cleaning effect was observed with no significant build-up of urea particles on the surface. Small deposits of dust that did remain were easily washed away with cold water. Although clouds of dust were formed they created no fire or other hazard and no ill effects to the operators were noted.

Example 3 In this example a chromated magnesium bearing carrier coated with Rockhard (an epoxy resin) and a green epoxy top coat was immersed in a solvent type paint remover operated at 100"C for a period of 3 hours. The softened paint was then removed by blasting with urea. The pressure was 80 psi (5.6 kg/cm2) with a nozzle orifice of 4 mm. Finally the surface was water rinsed to dissolve and hence remove final traces of the blasting medium. On areas not blasted with urea but only rinsed with water, removal of the paint scheme was incomplete.

Claims (10)

1. A process in which a surface is cleaned by dry abrasive blasting using particulate urea as the abrasive.

2. A process according to claim 1 in which the particulate urea has a particle size of 0.1 to 1 mm.

3. A process according to claim 2 in which the particulate urea is in the form of micropriils.

4. A process according to any preceding claim in which, after the dry blasting, the surface is washed with water to remove residual urea.

5. A process according to claim 4 in which the washing is effected by application of water to the surface by sprays or jets.

6. A process according to any preceding claim in which the dry abrasive blasting is conducted in a dry blasting cabinet.

7. A process according to any preceding claim in which the surface carries a paint coating or paint residues.

8. A process according to claim 7 in which the surface is treated with a paint remover before the abrasive blasting with urea.

9. A process according to claim 1 substantially as herein described with reference to examples 2 or 3.

10. An abrasive for dry abrasive blasting and that is particulate urea.