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标题: 一周年庆典－－SURFACE PREPARATION [打印本页]

作者: BMA004 时间: 2008-6-21 09:56
标题: 一周年庆典－－SURFACE PREPARATION
By the term surface preparation we understand all activities and methods involved in preparing metal surfaces for application of paint or a coating system. The specification on hand, or references to standards, will often determine the degree of preparation. To our disposal there are a number of methods and equipment and below is a discussion of such methods and the equipment involved.
Manual methodsMost people have, at some time or other, been involved in removal of rust by means of steel brushes or scrapers, pick hammers etc. Commonly the experience is that these methods are heavy, tiring and people quickly get bored from these methods. The efficiency is highly suspect and investigations have shown the following:
---a steel brush will remove approx. 5-10% of the total layer of iron oxide
---a steel scraper will remove approx. 30-35% of the oxide layer, through either thirty movements back and forth or forty or forty-five movements one way by the scraper
All taking into account that quite some pressure must be placed on the brush or
scraper to achieve this.
Scraping one small area forty-five times is not a viable prospect and certainly not worth the effort when taking into account that 65-70% of the oxide layer remains after this tremendous manual effort.
Equipment used for manual surface preparation methods are:
---steel brushes

Several types exist, with two, three, four and five rows of bristles. The types with a separate, raised handle will be more evenly worn than the ones where the handle is an elongation of the brush itself.
---steel scrapers

These scrapers come in many forms and shapes, with or without changeable blades. The most commonly used are the so-called angular scrapers, triangular scrapers and hard metal scrapers.
---pick hammers

These are hammers ending in a point and are used to hit the brittle layer of iron oxide thus removing portions of it.
---sand or carborundum paper

Both types are well known and it should be noted that carborundum paper last longer than sand paper and removes marginally more of the oxide layers.

Both sand and carborundum papers are invaluable when preparing the surface of existing paint for overcoating.
All the above methods are very slow and thus costly due to very limited production as surface area prepared per hour. Manual methods should only be selected when all other methods cannot be used and for small areas only.
The efficiency of mechanical methods is unacceptably low and these methods should be avoided both from a technical and an economical point of view.
Mechanical methods
These methods and tools make the work of removing oxides and impurities (partly or wholly) from the substrate less strenuous for the operators as they need less muscle power. The methods and tools mostly have the same drawbacks as the manual methods in as much as they are not very efficient in removing the oxide layer completely. They are also—despite being either electrically or pneumatically driven—slow.
---pneumatic hammers

These hammers can work as either a manual pick hammer would, or the hammer could be rotating. A special version is the rotating sling.
---needle-guns

This is a sort of pneumatic hammer where some needles have replaced the hammer (usually twenty to one hundred) collected in a bunch. The needles are rapidly hitting the surface cleaned from oxides and other impurities. With a needle gun having suitable needles, cleaning a steel surface to the point where it is suitable for application if inorganic zinc silicate is possible.
---scrapers

There does exist mechanically driven scrapers that move back and forth imitating the movement of the hand driven scrapers.
---grinders

These are rotating tools onto which are attached steel brushes (cup brushes), grinding disks of various forms and materials and grinding wheels. Some of these attachments can be quite effective in removing oxides, however, the speed at which they do so is still low.
Flame cleaningFlame cleaning is not very often used these days as most of the steel has usually been cleaned in a centrifugal blast machine and given a prefabrication primer (formerly called shop primer). Flame cleaning aims at the following:
---remove mill scale from the steel surface
---remove oxides or transform them into a less harmful state, from a corrosion point of view
---dry the steel surface
---burn off organic contaminants like oil and grease
---heat the substrate before application of paint
This method that we designates as thermal cleaning is done through passing a burner at a given speed and distance over the substrate to be cleaned. Mill scale and steel have different factors of expansion and the method utilize this fact. The tension created by the heat cause the mill scale to crack and flake off from the steel. The surface temperature should be approx. 150℃. At this temperature there will be no structural changes in the steel, however, the flame cleaning method should not be used on steel having thickness below 5-6 mm.
Blast cleaningBlast cleaning of steel, although employing mechanical means for the cleaning of the substrate, is classed by itself due to the special principle involved in the cleaning. Blast cleaning makes use of kinetic energy, whereas the mechanical methods described above are utilizing energy directly.
Blast cleaning involves throwing particles at a surface. In blast cleaning—whether this is air driven blast cleaning or blast cleaning by centrifugal force—the particles hitting the substrate have energy stored, which they release at the impact with the substrate. This energy is called kinetic energy. The formula for calculating this energy is as follows:

E = 0.5 m * v2

Where the kinetic energy (e) is the product of the mass of the particle (m) times the velocity of the particle (v) squared. The formula is used for calculating the efficiency of various abrasives (e.g. abrasives of different kinds with varying specific gravities) at given air pressures.
Besides the mass and the velocity, the shape of the particle and the length and type of blasting nozzle will have a definite impact of the efficiency of any blasting process.
Blast cleaning is mainly used for cleaning of metal surfaces, however, it is also used for cleaning moulds used for casting of metals, removal of paint from furniture, various decoration purposes for glass, ceramics, stone and concrete.
High tensile steels are blasted with round steel balls in a process called shot-peening to relive the steel from remnant tensive energy. The basis of the system is the same as what was done in older times by means of hammering, round shot is projected at high velocity on to the surface of the metal part being treated. The flow of shot causes a permanent stretching of the surface radially and causes a plastic flow of the surface fibers beyond their yield point in tension. The crystalline grains are reoriented over a zone that resists flow or fracture. The metal in this area is somewhat stronger than the metal below this zone.
Shot-peening may be applied to irregularly shaped parts where heat treatment may cause distortion. Shot-peening should be the final operation after machining, heat treatment, or grinding and only very mild surface treatments such as honing or very light sanding should be permitted finally.
Intelligent use of shot-peening has resulted in extended life of a wide range of machine parts. Examples of these are:

MACHINE PART TREATED:	INCREASE IN LIFE LENGTH
Crankshafts	900%
Leaf springs	600%
Connecting rods	1000%
Coil springs	1370%
Gears	1500%
Steering knuckles	475%
Rocker arms	1400%

Furthermore, abrasive blasting commonly clean aircraft engine parts and parts for turbines, however, the abrasive used is organic like, e.g. crushed walnut shells and cherry stones.
Blasting equipment
There are three main types of blasting equipment in use, and we classify these by their working principles:
---air pressure

In these apparatuses the abrasive is mixed into a high-pressure air stream from an abrasive container or hopper, through a mixing valve and a system of pipes or hoses. Finally the abrasive/air mixes exit the system through a nozzle and the abrasive particles are then projected onto the surface of the metal to be cleaned.
---water jetting

In these apparatuses water is the cleaning medium, although abrasive particles may be injected into the water stream. The water is projected onto the surface at very high pressures. Water washing/jetting is usually classified as follows:

CLASSIFICATION:	WATER PRESSURE AT NOZZLE:
Low pressure water washing	< 34 Mpa (5000 psi)
High pressure water washing	34--70 Mpa (5000-10000 psi)
High pressure water jetting	70-170 Mpa (10000-25000 psi)
Ultra high pressure water jetting	> 170 Mpa ( > 25000 psi)*

l
= Most machines in this classification operate in the 2000-2500 bar 200-250 Mpa range (30000-36000 psi), however, equipment for ultra high pressure water jetting at pressures of 300 Mpa (45000 psi) exist.

At the moment there is a very rapid development in the field of water jetting equipment and higher water pressure levels will be seen within short.

There has also been a rapid decrease in the amount of water used by the equipment and presently the below is the situation for the two dominant pump types used:

TYPE OF PUMP	NOZZLE PRESSURE	WATER CONSUMPTION
Triplex plunger	200 Mpa (30000 psi)	20 l/min
Linear duplex	300 Mpa (45000 psi)	6 l/min

The latter consumption volume opens the way for hydro blasting aboard vessels during voyage, as most on-board evaporators will be able to furnish the water needed.
---centrifugal

This equipment, which is often called wheel abraders, use centrifugal forces for projecting an abrasive flow onto the surface. The abrasive is fed to a fast rotating wheel through the axle and then in the wheel to the perimeter where the abrasive is released through an opening onto the substrate inside an enclosed chamber. The steel is conveyed through this chamber, thus exposing all areas to this abrasive “rain” as the centrifugal wheels are placed both above and below the conveyor (or alternatively on the sides).
Both wheel abrading and air pressure driven blasting not only remove oxides and (partly) other impurities from the substrate, these methods also give surface roughness (or anchor pattern as the Americans call it). High pressure water jetting does not create any surface roughness at all on metal surfaces, which limits this cleaning method for use in removing old paint from surfaces that already have the needed surface roughness or to cleaning surfaces that will not later require a surface roughness. Surface roughness is more properly designated surface configuration as this term covers not only the surface profile but also the amount of features per area unit existing on the substrate.
Abrasives
Originally natural sands were used for abrasive blasting, which in those days was called sand blasting. Fresh water sands were used as these were free from chlorides and in the US the famous Ottawa Sand became the industry standard. In Norway, for tank blasting, the equally famous Wollstad Sand became the industry’s yardstick. Sand was used successfully for decades, however, due to the danger of personnel being exposed to the dust from the sand, which contained free silica and thus could cause silicosis, the use of sand with free silica was either forbidden or voluntarily discontinued.
Other blasting media came into use, however, one should note that although these do not contain free silica, protection against the dust must still be provided.
Blasting abrasives, which are angular in shape, are called grit and those, which have a round shape, are called shot. We classify abrasives by their generic type, their shape, their range of particle sizes and their hardness. Abrasive suppliers should deliver products in line with the ISO 11124—11127 standards.
Numerous, widely varying types of blasting media are on the market and the below table should cover the most common ones:

作者: BMA004 时间: 2008-6-21 09:58
GENERIC GROUP: NAME OF ABRASIVE:
NON-METALLIC: Minerals, natural: Silica sand
Olivine sand
Garnet sand
Zircon
Emery
Minerals, artificial: Coal slag
Aluminum silicate
Iron slag
Copper slag
Aluminum oxide
Ceramic glass
Silicone carbide
METALLIC: Steel (shot, grit and wire cut)
Chilled iron (shot and grit)
Aluminum
Bronze (shot)
ORGANIC: Polystyrene beads
Nylon pearls
Walnut shells (ground)
Cherry kernels (ground)
Olive kernels (ground)
Rice hulks (chopped)
ORGANIC: Coconut shells (ground)
Almond shells (ground)
Corn cobs (crushed)
Oat husks (ground)
OTHERS: Water
Ice crystals
CO2-ice
Baking soda
Sponge covered
Zinc covered

From the above it can be seen that a variety of blasting media exist to cover whatever the need might be.
Some of the above-mentioned abrasives like, e.g. steel shot and grit, may be reused after cleaning and dust separation, however, the majority of abrasives are expendable and are disposed of after use.
Abrasive blasting
This term is usually connected to the air driven blast cleaning. There are two main types used, namely dry abrasive blasting and wet abrasive blasting. The latter method is split in two subcategories—wet blasting and slurry blasting.
For all the above methods the equipment used is essentially identical and consists of the following:
Compressed air flow
---A source of air supply with sufficient capacity to run the equipment coupled to this air source is needed. If working at a site with several other trades coupled to the same air source it will usually prove advantageous to have a separate air source for the blasting process as fluctuations in a large system with multiple users will be too great. Abrasive blast cleaning is dependent on stable supply of air at a minimum pressure and in sufficient volume, none of which can be guaranteed in a larger network.
---The compressed air is normally fed into an expansion tank where fluctuations in the pressure are leveled.
---From the expansion tank the compressed air is led to the blasting pot where the hopper part of the blasting pot is kept at the same pressure as is in the system through the air being passed both through the hopper and in a pipe bypassing the hopper. Thus the dry abrasive will fall by normal gravitational force through a dosage valve (sometimes called a miser valve) into the main pipe where the compressed air flows towards the hosed and the blasting nozzle.
---It is necessary to carry out regular checks on the compressed air to find out whether or not water and/or oil are present. There are normally water and oil traps on the compressor, the expansion tank and the blasting pot, and these must be checked and emptied regularly. Water or oil in the air may be checked by holding a sheet of, e.g. brown wrapping paper in front of the airflow. Water and oil will show as fine spots on the paper. The water spots will dry after a while, whereas the oil spots will not.
---For all supply of compressed air the rule is to have pipes (or hoses) of sufficiently large diameters. Commonly one sees pipes and hoses of insufficient bore being used for air supply. Remember that working with air as the energy source it is not only necessary to have sufficient pressure, volume of air is equally important.
The blasting pot
---The pot itself is of a simple construction, consisting of a hopper for the abrasive (usually 80-100 l capacity). At the inlet for compressed air the pipe is divided with one part leading into the abrasive hopper through a manometer and the other one joining with the dosage valve from at the bottom of the hopper. One pipe then leads the abrasive/air mix to the hoses. The hopper is filled with abrasives from the top. When the air pressure is turned on, a stopper closes the opening in the top of the hopper causing the interior to come under pressure. The pressure in the hopper is equalized to the pressure in the bypass pipe. In this way the abrasive will fall down into the air stream by gravity when the dosage valves is opened. The air stream will bring the abrasive/air mix through the hoses (usually thick walled made from reinforced rubber) to the blasting nozzle.
---The hoses are normally made in standard lengths with external, quick-couplings leaving a minimum of obstacles inside the hose.
The blasting nozzle
There are a multitude of nozzles available, however, two main types are dominating namely the straight bore and the venturi types. Nozzles are usually made from steel lined with a tungsten carbide, however, ceramic and plastic (PVC with tungsten carbide lining) nozzles are also in use. The latter types are relatively inexpensive, however, quite brittle and will easily chip or crack if knocked against the substrate.
The length and bore of a nozzle are vital to its efficiency and overall it should be observed that longer nozzles with a large bore are more efficient (as calculated in m2 cleaned per time unit) than shorter nozzles with a smaller bore.
The venturi nozzle has an internal diameter that narrows down approx. halfway down its length, and then increases in diameter from that point. The object of this design is to increase the airflow by that increasing the velocity of the abrasive. A correctly designed venturi nozzle in which the convergent inlet has been calculated precisely, with the throat and divergent area of the necessary accuracy, will produce a supersonic airflow at the exit point. The venturi accelerates the airflow, and according to Bernoulli’s Law, this high velocity air creates a low-pressure region that further accelerates the abrasive/air mix. In addition the abrasive will flow more evenly causing less wear on the tungsten carbide lining in the nozzle.
Centrifugal wheel plants
These cleaning machines are usually found where there is a need for blast cleaning of a large number of simply shaped, like-formed objects. A typical example is blast cleaning of, e.g. steel plates and profiles. Such centrifugal cleaning machines or plants are normally part of what we call prefabrication (or shop) priming plant where the objects after cleaning receive a thin coat of a prefabrication primer (also called a shop primer).
---In a plate priming plant, the plates are brought onto the conveyor from the storage area. The plates first pass through a preheating area where the temperature of the plates is brought to the desired level. Such preheating is usually done with above and below burners running on gas or oil, however, electrical heaters have also been used.
---The plates then enter the wheel-abrading machine. The plate size will determine the number of wheels, however, six to eight wheels are normal. The steel is cleaned through the impacts from the abrasive particles, which are flung onto the surface on both sides of the plates. Exiting the wheel abrader, the plates are brushed and blown with air to remove remnants from the abrasives. The abrasive/dust mix is then transported through a cyclone cleaner where particles below a preset diameter are removed, back into the plant’s hoppers for reuse. This is called recycling of the abrasive.
---Abrasives used in a wheel abrader machine are usually steel shot or grit, chilled iron shot or grit or wire cut. The latter is the least efficient of these, whereas the grits are the most efficient ones, leaving the best surface profile on the substrate. However, grits cause heavy wear on the centrifugal wheels (unless these are lined with tungsten carbide). Chilled iron shot and grit as well as steel grit have an angular shape when in use, which is beneficial to both the efficiency of cleaning as well as the production of a desired profile.
---Finally the plates pass through the painting booth where automatic spray guns pass over the surface depositing a thin (13—25 μm dry film thickness depending on the generic type) coat of the prefabrication primer. The function of this primer is to protect the steel from corroding during the fabrication period. A prefabrication primer thus is intended as temporary protection only, except in the case of the inorganic zinc silicate prefabrication primers as these types can be an integral part of an effective, long lasting corrosion protection system based on paints.
High pressure water jetting
As previously stated, high pressure water jetting does not produce any surface profile on metal substrates. However, this cleaning method is highly efficient on surfaces, which previously have received a surface configuration, which is suitable. In addition, high pressure water jetting very efficiently removes any water-soluble contaminants from substrates and may in repair situations eliminate fresh water washing, thus saving time and money.
High pressure water jetting is very well suited for removal of e.g. old layers of paint and concrete from metal substrates. It should be noted that this method has its limitations when used in enclosed spaces like tanks, as the air will become quickly saturated with water vapour exposing workers to problems with breathing and ultimately drowning, as well as problems with seeing the substrate through the water fog.
Parameters of efficiency
The following table lists the various parameters, which must be taken into account for obtaining maximum efficiency of blast cleaning processes:

PARAMETERS: DESCRIPTION OF FACTORS:
---Generic type of abrasive
---shape of abrasive
---hardness
---strength
---dust formation at impact
---composition
---particle size and distribution
---occupational health hazards
Cleaning: ---air pressure and volume
---compressor size
---nozzle size and type
---equipment type and size
---nozzle handling (distance and angle)
---abrasive/air mix
---energy consumption
---dust generation and visibility
Substrate: ---metal quality and type
---thickness and hardness of mill scale
---degree of corrosion
Substrate: ---contaminations
---thickness of metal
---shape
Cleanliness: ---physical cleanliness
---chemical cleanliness
---degree of metal cleanliness required
Configuration: ---degree of profile required
---surface configuration needed
Others: ---facilities on hand (e.g. blasting sheds)
---access to substrates
---climatic conditions

Taking the above parameters into account, blast-cleaning processes may be greatly facilitated and increased in quality, at the same time lowered in cost.

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