blasting procedure

8/19/2019 Blasting Procedure

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BLASTING PROCEDURE

FAISAL S. AL‐NAIMI

Al‐Jubail 31951, Kingdom of Saudi Arabia


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ABRASIVE BLASTING

WHY IS SURFACE PREPARATION IMPORTANT?

Surface preparation is the most important part of a coating system, because it affectsthe performance of the coating more than any other variable. Given that the correctcoating system is selected, if the surface preparation is poor, coating performance isusually going to be poor. If surface preparation is good, then the coating applied overit is likely to perform well.

It is useful to know the reasons why surface preparation is so important, becauseknowing “why” can help the applicator do a better job.

SURFACE PREPARATION AS A FOUNDATION

Surface preparation is to a coating system what a foundation is to a building. If abuilding has a poor foundation, it can list or lean, as the famous Leaning Tower ofPisa does, or it can collapse altogether. If a coating system has a poor foundation(surface preparation) it will fail sooner than expected (say, after five years rather than10 years); or it can fail catastrophically within the first year of application. In bothinstances great financial losses can occur to a facility owner. Surface preparationcreates a foundation in two important ways:

*a mechanical way, by providing an anchor for the coating; and* a chemical way, by allowing intimate contact of coating material molecules with thesteel (or other material) surface.

WHY AN ANCHOR PATTERN?

When a surface is very smooth, coatings have a difficult time adhering strongly. Ascraper or even a fingernail, for instance, easily removes a coating on glass. On theother hand it is difficult to remove a coating on a rough surface like sandpaper. Steel,when it is abrasive blasted, has a surface that is rough like sandpaper, with a seriesof tiny peaks and valleys called surface profile. Coatings anchor themselves to thevalleys of the profile, and the peaks are like teeth. This is why surface profile createdby blasting is sometimes called an "anchor pattern" or "mechanical tooth."


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SURFACE SOILS

- Visible contaminants

Soils on a metal surface are made up of many different materials. They include:* Oil* Grease* Corrosion products* Oxides/Mill Scale* Perspiration* Marking pen ink* Rubber from boots* Brazing flux* Weld flux* Weld scale

* Weld anti-spatter compound* Adhesive from tape* Dirt/dust* Chemicals/Salts* Smut* Metal chips* Drawing compounds* Polishing/buffing compounds* Abrasive* Mould release agents* Previous organic coatings* Previous metallic coatings* Finger prints

* Silicone.

When contaminants such as these are painted, they interfere with the mechanicaland chemical adhesion of the coating to the substrate so that the coating is likely tofail. On the other hand, when all soils are removed, the coating can achieve completeand continuous contact with the substrate, thus assuring the best possible adhesion.When a coating adheres well, it is likely to be an effective barrier. The coating canminimize or prevent moisture (the electrolyte in the corrosion process) from reachingthe substrate.

-Non-visible contaminants

Other forms of soils, not always visible to the naked eye, are chemical contaminants.

The most dangerous forms of chemical contaminants are soluble salts:* chlorides and* sulphates.

When such contaminants are painted over, they have the power to draw moisturethrough the coating (osmosis) to cause blistering, detachment, and acceleratedcorrosion of the underlying metal. When structural steel is repainted, rough or pittedareas visible after dry abrasive blast cleaning may contain soluble salt contamination,especially in the base of the pits. Dry abrasive blasting does not remove these salts.


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It is wise to check for the presence of soluble salts with specially designed field testkits before painting and if they are present in detrimental amounts, to take additional

cleaning steps to remove the salts.

DEGREES OF SURFACE PREPARATION

In any job specification, the degree of cleaning required for a given substrate beforepainting depends on a number of factors.

* The service environment of the coating system. This is perhaps the most importantfactor, and normally is the first consideration when determining the degree of surfacepreparation. Generally, the more severe the environment, the better the surfacepreparation required. Severe service environments include:

- Immersion in liquids,

- Exposure to aggressive chemicals or environments, and

- High temperatures, or combinations of these conditions.

* Another consideration is the generic kind of coating used. Some coatings, such asalkyds, because they flow out and wet the surface well, can tolerate application overminimally prepared or hand-cleaned surfaces. In addition, some epoxy mastics andother "surface-tolerant" coatings are formulated to be applied over hand-and powertool-cleaned surfaces. Coatings such as vinyls and inorganic zincs, however, are atthe other end of the spectrum. They require a higher degree of cleaning than manyother types.

* Cost is another factor when selecting the degree of surface preparation. Blastcleaning to Class 3 (White Metal) is about 4-5 times more costly than to Class 1(Light Blast Cleaning). In some severe environments and with some coating types,rigorous cleaning is necessary; but in other instances, cost and cost-benefit in theform of longer coating lifetime will become an important factor in selecting the degreeof surface preparation.

* Finally, regulations may have an impact on the degree and method of surfacepreparation. - in residential or congested urban environments, open blasting may beprohibited and - in addition, where lead-or chromate-based paints are beingremoved, hazardous waste regulations may require containment and use of special

surface preparation methods.

Determining the degree of surface preparation, as described above, is the job of a

specifier or engineer. The task of doing the work is the contractors. No matter what

degree of surface preparation is required, it must be done thoroughly. If hand-tool

cleaning is required, then all the surface area specified must be hand-tool cleaned,

after it has been cleaned by water or solvent to remove dirt, oil, or grease. Similarly, if

a Class 3 blast is specified, then conformance with the written description of this


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must be achieved on all surfaces. When preparing metal, it is also important to follow

the proper sequence.

* First, remove dirt and other soils. It is a lot easier to sweep mounds of dirt and other

loose material off a surface with a broom than to try to remove it with surface

preparation tools.

* The next step is removing visible oil and grease by solvent cleaning. Then conduct

the mechanical cleaning operation whether hand tool, power tool, or blast cleaning. If

these steps are reversed, particularly with blast cleaning, the force of the blasting

abrasive can drive the soils into the roughened steel surface or profile. Then it is not

easy to remove, and it may interfere with coating adhesion. In addition, it is important

to achieve the surface profile required by the specifications, because:

- When the profile is too rough, the coating may not cover the peaks of the profile,

and the result will be pinpoint rusting.

- When the profile is not rough enough, the coating may not anchor well to the

surface, and the result will be loss of adhesion. To make sure that a coating system

will perform well as a barrier to prevent/reduce corrosion, all soils must be removed

so that the coating contacts the entire surface of the metal for chemical adhesion and

that the

Surface is roughened for mechanical adhesion as well. These two conditions ofcleanliness and profile ensure that a proper foundation has been created for applyingthe coating system. This good foundation should help to provide many years ofservice life for the coating.

HOW CLEAN IS CLEAN?

Protective coatings have been used for centuries to protect substrates subjected tothe environment. The most severe environments are near the ocean. Many industrialplants have severe exposures that require protection.

For years, most coatings were applied by brush to hand-cleaned surfaces. The fishoils and long and medium oil alkyd coatings were pigmented with some very effectiveinhibiting pigments such as red lead, and they were literally scrubbed into the surfaceby the brushing action. These materials were good at sticking on the surface, andthey could be applied over mill scale and tight rust to provide some protection.

Of course, they would protect longer if they were put on a blast-cleaned surface.Many of the new, high performance coatings do not stick well to mill scale, tight rust,or a smooth surface. Abrasive blasting is needed to clean and roughen the surface.


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.....Abrasive Blasting Media -Garnet .....................Glass Media

* Abrasive blastingthere are four (4) levels of abrasive blasting described in AS1627-Part 4. Thefollowing lists them in increasing levels of cleanliness:- Class 1

- Class 2- Class 2 1/2- Class 3.These specifications define the physical cleanliness that must be achieved on thesurface.- Cleanliness The last three of these specifications requires all the mill scale, rust,and old paints to be removed. All that can remain on the surface are stains of thesecontaminants.

The following amount of stains is allowed (by visual estimation).

· Class 2 - 33 percent of each sqcm,

· Class 2 1/2 - 5 percent of each sqcm

· Class 3 - none.

The most common tools used in the industry to assist in determining surface

cleanliness are

· SSPC - Vis 1-89,· NACE coupons, and· Swedish pictorial standards as specified in AS1627.9

- Anchor profile The height of the anchor profile is specified independently fromcleanliness. The manufacturer's application data sheet will give this information.There is no standard anchor profile height that is good for all coatings. The surfacemust be roughened sufficiently to get the coating to stick. Coatings that are applied inthin coats, such as oil-based coatings, require a low anchor profile ie, 50 microns.Too heavy an anchor profile will result in the peaks of the profile in the steel stickingout and causing pinpoint rusting. Thick coatings such as coal tar epoxies require adeep anchor profile ie, 100-125 microns, to get them to stick properly. Two commontools are used to determine anchor profile:


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· A surface profile comparator and

· Replica tape.

ASSURING QUALITY OF ABRASIVE BLASTING OPERATIONS

The performance of a coating depends in large part on the quality of surfacepreparation. This is because coatings have been formulated to perform properlyunder particular conditions, such as over a specified degree of surface cleanliness

and a specified anchor profile, and under certain environmental conditions. If theseand other conditions are not met, coatings may not achieve their expectedperformance. When dry abrasive blasting is the specified method of surfacepreparation, many conditions must be taken into consideration by the blaster.

* Checking conditions before blasting

Most coatings do not adhere well to surfaces contaminated with oil and grease.Blasting actually drives them further into the steel rather than removing thesecontaminants and thus contributes to premature coating failure. Therefore, a checkfor visual surface contaminants before blasting is essential. If oil and grease arepresent, they should be removed with solvent cleaning, in accordance in AS1627-Part 1.

All of the blast cleaning specifications from AS1627-Part 4 require this step with thestatement, Before blast cleaning, remove visible deposits of oil or grease by any ofthe methods specified in AS 1627 Part 1.


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Ambient conditions should be measured before blasting. If blasting is not to befollowed immediately by coating application, then it may be all right to proceed firstwith rough blasting to remove the existing coating, rust, and mill scale, and to checkambient conditions before the final blast. If blasting is to be followed immediately by

coating, then ambient conditions should be checked before blasting begins.

It is essential that the dew point, air temperature, relative humidity, and surface

temperature are suitable for blasting. This insures that condensation will not be

forming on the metal surface during or after blasting and cause flash oxidation (rust),

which can be detrimental to the overall quality and coating performance. Dew point is

the temperature at which moisture condenses on a surface. If the dew point is 10C,

condensation will occur if the metal is at or below this temperature. As a general rule,

final blast cleaning should take place only when the surface is a least 3C above the

dew point. For example, if the dew point is 10C, the steel temperature should be at

least 13C. This rule provides a margin of error, in case of instrument inaccuracies,

quickly changing weather conditions, or human error. Dew point is calculated usingthe relevant psychrometric tables. The psychrometer is a hand-operated instrument

that has 2 glass thermometers.

To measure ambient conditions, the following equipment is required:- a surface temperature gauge or surface thermometer.

- a psychrometer for measuring dry bulb (air) and wet bulb temperature, andpsychrometric tables for calculating dew point and relative humidity.

* Checking blasting abrasives and equipment

Abrasives and equipment should also be checked for cleanliness before blasting, andthe equipment should be checked for efficiency. There are several parts of theblasting equipment that need to be checked for contaminants:

- The compressor,

- The moisture separator, and

- The air that comes through the hoses.

WET ABRASIVE BLASTING

Wet abrasive blasting means abrasive blasting where water is added to the abrasivematerial or its propellant or is used as a propellant. Whilst the addition of watershould reduce or minimize the risk of exposure to dust, other hazards associated withabrasive blasting will remain and have to be controlled.

The same processes and procedures that apply to dry blasting apply to wet abrasiveblasting.


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While a distinction can be drawn between wet abrasive blasting and the use of waterat high pressure or water jetting of material to prepare surfaces, similar requirementsrelate to the plant used in both processes.

Because of the high pressures used, measures should be taken to ensure the safetyand health of all persons, whether operating the equipment or in the vicinity, areprotected.

Only high pressure hoses, firmly secured, should be used and:

All bypass valves should be equipped with pressure safety relief valves;

A funnel should be fitted near the end of the nozzle to minimize the risk of thewater stream hitting the operator ; and

An automatic cut-off device (deadman control) should be fitted to the nozzle.This device should be capable of being activated when the start switch isreleased if, for example, the operator accidently drops or loses control of the

nozzle.

This is particularly important if the equipment is being used:

In a confined space;

Above ground level; or

Where the wet blasting operator may be temporarily out of sight of anobserver.

A deadman control should only be used according to the manufacturer’s instructionsand not disabled or removed to allow continuous function.

If the nozzle is not fitted with a deadman control the employer must ensureprocedures are in place that will allow a person other than the person operating thenozzle to cut off the flow if the person operating the nozzle is unable to do so.

All hoses or lines should be positioned in locations where they are not subjected todamage, fouling or restrictions such as from vehicles, building or fences.

Personal protective equipment to be used when operating water blasting or water jetting plant includes protection for the:

Head;

Eyes;

Body;

Feet; Hands;

Ears; and

Respiratory system.

Rust inhibitors containing chromate, nitrate or nitrite are not to be used whenwet blasting.


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* Checking the surface after blasting

After blasting, remove all dust from the blast-cleaned surface, either by blowing downthe surface with compressed air or by vacuuming. Dust on the surface can interfere

with the coating's ability to bond to the surface. After blowing or vacuuming thesurface, brush a clean white cloth across the surface (without touching the substratewith the hands as body oils or salts can be transferred easily to the surface andcontaminate it). If dust appears on the cloth, blow down or vacuum the surface again.Check for non-visible contaminants, especially soluble salts, which are detrimental tocoating performance. Once the blasted surface is free of dust (and othercontaminants), check the surface profile and degree of cleanliness to see that thespecifications have been met.

RECORD KEEPINGThe quality control checks should be documented and kept as partof the quality control records for the job. This way, historicalinformation is available for verifying compliance withspecifications.

CONCLUSION

There's an old cliche that a top quality coating put on a marginally

prepared surface will perform no better than a cheap coating applied to a squeaky

clean surface. This statement is probably an oversimplification of the problem, but

there's a great deal of truth in the statement as well.

Surface preparation is an important step that affects the life of a coating. The life of

an oil-based paint, for example, is longer on a blast-cleaned surface than on a hand-

cleaned surface.

Many of the high technology coatings such as zinc-rich primers require a blast-

cleaned surface to stick and provide protection to the steel. Coatings stick better to a

rough surface than to a smooth surface.

Abrasive blasting both cleans a surface and roughens it. This roughness is called

anchor profile. The specification will specify how rough the surface must be before

the paint is applied.

Quality assurance will help ensure that abrasive blasting operations create a surface

suitable for coating application and should be followed even if customer's inspectors

are on the job conducting similar checks.

blasting procedure

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