Hand tools

Hand tools are no longer commonly used for professional surface preparation. They require muscle power and plenty of time and patience. In return they do not give a very good standard of preparation.  Their main advantages are that they are easy to transport and are not depending on any power supply. Examples of hand tools used for steel preparation:

• Scrapers, used to remove rust and old paint
• Wire brushes, used to remove rust
• Hammers, used to knock off / chip flakes and cakes of rust
• Emery paper and abrasive pads, used to grind rust and old paint

Power tools

These can either be driven by electricity or by compressed air (pneumatic). They are easier to work with and have a higher production rate than hand tools, but the quality of the output has still some limitations. Most of them are working on a rotating basis or are reciprocating impact tools. Examples of power tools used for steel preparation:

• Rotating wire brush, used to remove rust
• Rigid disc grinder, used to grind away steel defects (faulty welds, spatter, delamination, etc.), sharp edges, and rust and mill scale
• Flexible sanding disc, used to abrade old paint and general rust, feather edges of old paint
• Needle gun, removes rust scale
• Chipping hammer, removes rust scale
• Rotary impact or scarifying tools, such as Roto-Peen and Bristle Blaster, used to remove rust

Most of the power tools are hand-held portable type, but there are some larger versions as well of the “walk-behind” type

Hand- and power tooling

Advantages

• Simple
• Good for poorly accessible areas
• Cheaper

Limitations

• Low productivity
• Rotating wire brushes and disc-sanders may give a polished surface
• Needle gun may create indentations and pits - hard to paint
• High labour cost

Standard

• ISO 8501-1 St 2 and St 3 surface condition can be achieved
• Manual wire brushing is heavy work and has very low productivity
• Rotating wire brushes and disc grinders speed up the production rate but increase the risk of having a polished surface
• A polished substrate will result in poor adhesion of the paint

This is the most common surface preparation method for large areas. The surface is cleaned by propelling abrasive particles at high speed towards the surface to be treated. The impact of the abrasive particles will remove rust, mill scale and old coatings.

The abrasive can be propelled by:

• Compressed air
• Centrifugal force
• Water with high velocity
• Combination of Air and Water (slurry blasting / vapour blasting)

Compressed air abrasive blasting

Abrasive is fed into a high-pressure air stream from a pressurized container through a mixing valve to a hose which is used to direct the air/abrasive mixture at high velocity from a special nozzle toward the surface to be cleaned

Advantages

• High productivity
• Creates a surface profile which can be varied
• High degree of surface cleaning at high speed
• The most efficient method of surface preparation
• Surface remains dry
• Good anchor pattern for paint
• Flash rusting avoided

Limitations

• Complex equipment
• May need dehumidification of the blast air
• Pollution (dust, heavy metals, etc.)
• Noise
• High velocity stream of abrasives is a safety risk
• Does not remove salt
• Does not remove oil
• Creates dust

Abrasive blast cleaning produces a good surface profile and several visual blasting standards can be achieved, but it does not remove soluble salts from the substrate.

Main components

• Compressed air
• Abrasives
• Blasting pot
• Metering valve
• Blasting hose
• Control hose
• Dead man’s switch
• Blasting nozzle

 

Blast pot

The blasting pot is pressurised by the same compressed air which drives the abrasive. A pop-up valve at the top of the pot is opened when more abrasive is added to the pot. The pressure in the pot prevents a “blow-back” when the abrasives are gravity-fed into the airstream through the metering valve.

 

Workers filling blast pot

 

Productivity

In order to achieve the best efficiency, the mixture ratio of abrasive and compressed air is a key factor:

• Too much abrasive: slow efficiency, low profile and waste of abrasive
• Too little abrasive: Slow efficiency as the time to achieve the required result is extended for each m² and abrasive can be wasted
• The ratio of abrasive to air is adjusted by the metering valve under the blasting pot

 

Productivity is directly proportional to the pressure at nozzle:

• Pressure at Nozzle 7 kg/cm²  = 100% productivity
• Pressure at Nozzle 5,6 kg/cm² =   66% productivity
• Pressure at Nozzle 4,2 kg/cm² =   50% productivity

 

The Blasting Hose & Nozzle

• The blasting hose is made of special pressure and abrasive resistant material and can be tiring to work with due to its weight and stiffness
• When extending the hose, it is important that the couplings are of approved type and without damages
• To minimize loss of pressure, the blasting hose shall have a diameter which is 3-4 times bigger than the diameter of the nozzle
• Blasting efficiency is limited by nozzle size, bigger nozzles consume more air
• Nozzles will be worn with regular use
• Once the nozzle size exceeds the manufactures limit (normally 1 – 1,5 times), it should be replaced
• Two main types of blasting nozzles:
  • Standard nozzle abrasive speed: 300 Km/h at 7 bar pressure
  • Venturi nozzle abrasive speed: 700 km/h at 7 bar pressure

 

Spot blasting

During repair and maintenance work, some owners may specify that only corroded spots and areas shall be blast cleaned, often referred to as spot blasting, and leave intact paint untouched. However, nearby paint and all edges between blast-cleaned spots / areas and remaining paint will be damaged by stray abrasives from the blasting. The edges will be rough and clearly suffering from impact damages (punctures). Additional damages, which are not easy to see, will be small cracks in the remaining paint (micro-cracks).

 

When the remaining paint is overcoated by new paint, solvents will penetrate these cracks and cause a reduction in adhesion near these edges, sometimes also a slight lifting of the old paint. The result is that after a relatively short time the blasted spots may still look good, but the edges of the spots suffer from new corrosion (and the owner complains about a poor blasting and painting job).

 

In order to avoid this disappointing result, all edges of blast-cleaned areas must be feathered by grinding equipment before being overcoated. Feathering is both time consuming and costly, so the best solution is to blast-clean larger defined areas instead of many small spots.

 

 

Abrasive types

There are two main groups of abrasives used for blast cleaning:

• Metallic abrasives:
  • Chilled iron grit (angular)
  • High-carbon cast steel grit and shot (angular or round)
  • Low-carbon cast steel shot (round)
  • Cut steel wire (cylindrical)
• Non-metallic abrasives (Mineral and slag abrasives):
  • Natural abrasives: Garnet, Olivine, Staurolite
  • Slag: Copper refinery, Nickel refinery, Coal furnace, Fused aluminium oxide, Iron furnace

The shape of abrasive particles influences the surface profile. Three dominant shapes of abrasive:

• Angular (grit)
• Round (shot)
• Cylindrical (wire cut)

Abrasive are selected to achieve the specified roughness and cleanliness:

• Particle size will define the depth of the surface profile
• Distribution of abrasive will define the surface profile and peak density
• Abrasive must be free from contaminants such as oil, salt or other
• The choice of abrasive can also be influenced by:
  • Type of substrate
  • Productivity required
  • National / International laws and regulations

Quality control of abrasives

• ISO 11124 gives specifications for metallic abrasives (Chilled-iron grit; High-carbon cast-steel shot; Low-carbon cast-steel shot; Cut steel wire)

• ISO 11126 gives specifications for non-metallic abrasives (Silica sand; Copper refinery slag; Coal furnace slag; Nickel refinery slag; Iron furnace slag; Fused aluminium oxide; Olivine sand; Staurolite, Garnet)

Re-use of abrasives-recycling

• Metallic abrasives, Carborundum and Aluminium oxide can be recycled and are usually used more than once, particularly for stationary plants
• Mineral and slag abrasives are generally used only once and in open blast-cleaning

Centrifugal (wheel abrader) blast cleaning

Two types:

• Mobile units
• Fixed installation

Abrasive is fed to rotating wheels or impellers positioned to throw the abrasive evenly and at high velocity on to the surfaces to be cleaned.

Mobile units

Mobile centrifugal blasting units can for example be used on steel deck (ships) and concrete floors (buildings). The equipment is heavy and only suitable for large, flat areas and has a relative high investment cost. It is environmentally friendly since it reclaims abrasive & dust, but the blast profile can become lower due to the recycling of the abrasives.

Fixed Installation

A system typically consists of:

• Abrasive throwing wheel (or impeller), the number of wheels can vary
• Blast cabinet
• Work piece handling equipment
• Abrasive recycling system
• Dust collector
• Abrasives

The wheel or impeller is the most important part of the system. The wheel or impeller can:

• Accelerate the abrasive by centrifugal force to “throw” the abrasive randomly or it can be aimed specifically so that complex shapes can be treated
• Use a mix of grit and shot for greater efficiency and better surface profile
• The more wheels used can allow for high efficiency and a variety of complex shapes to be cleaned continuously

Factors that can influence the end surface cleaning result:

• If set to the wrong angle the abrasive can miss the target
• The higher the rotating speed the higher the throwing speed and impact force of the abrasive
• These machines are subject to high wear and require frequent maintenance
• Abrasive wears during use and needs to be topped up to maintain a constant quality and surface profile

Advantages

• Very high productivity
• Very little dust produced when operating in open air
• As the motor is electric there is no need for a compressor
• Cost saving in the long term

Limitations

• High initial cost
• Complex equipment
• High maintenance – Worn parts
• Number of wheels can limit the size and shape of treated structure

Methods of wet blasting

Classification of wet blasting depends on how the water and abrasive is mixed:

• Slurry and Vapour blasting:
  • Abrasive and water mix together in a vessel and spray through a blasting nozzle
• Abrasive injection blasting:
  • Water is highly pressurized and sprayed while a separate nozzle is used to inject abrasive into the water stream in front of water nozzle
• Abrasive blasting with a water shroud:
  • A water shroud unit is placed at the end of the blast nozzle to form a water mist in front of the abrasive nozzle to wet the abrasive as it leaves the nozzle

Wet blasting

Advantages

• Eliminates most dust
• Removal of soluble salts and other contamination in one process
• Reduces damages to adjacent areas from abrasive rebound

Limitations

• Depending on the method selected, flash rusting can occur rapidly after blasting
• Inhibitor may be required to control flash rusting
• Not as efficient as dry blasting
• May be necessary to remove remaining abrasives by hosing down

Vapour Blasting

• This is the newest method of dust control for abrasive blasting
• Water and abrasive are mixed in the blast pot in precise (computer controlled) ratios so that the water level is only enough to reduce dust and not flood the surface with water
• Nearly the same productivity as dry blasting
• Flash rusting is not an issue – residual water flashes off

Vacuum blasting

• This equipment has a vacuum attachment which collects the spent abrasive as you blast:
• Equipment is usually for small repairs
• Several types of nozzles profiles can be used to make it possible to treat all steel profiles
• All types of abrasives can be used
• Possible to recycle the abrasives
• Handy size
• Slow production rate
• Environmentally friendly

Cleaning with water has the obvious advantage over abrasive blast cleaning that it does not require a large clean-up operation of spent abrasives and has no abrasive cost. It will however make the substrate wet and it will not create any surface profile.

Definition of water cleaning & water jetting

Both standard ISO 8501-4 and NACE NO. 5/SSPC SP-12 give the following definition:

• Below 70 MPa, the techniques are called water cleaning, classified as:
  • Low-pressure water cleaning (LPWC), defined as cleaning performed at pressures less than 34 MPa (5,000 psi)
  • High-pressure water cleaning (HPWC), defined as cleaning performed at pressures from 34 MPa to 70 MPa (5,000 psi to 10,000 psi)
• Above 70 MPa, the process of cleaning is generally described as high-pressure water jetting
• Above 200 MPa (30,000 psi), the term Ultra-High Pressure Water Jetting (UHPWJ) is commonly used

Surface preparation results using water

• Water cleaning may only remove oil/grease, dust and loose contamination
• Water jetting cannot remove mill scale, but it does remove rust, weathered coatings, etc.
• Ultra-high water jetting equipment is now available which can reach nozzle pressures of 600 MPa (90,000 psi). This gives very high productivity and cleaning efficiency, but the process has safety issues that must be recognised and planned around

Ultra-High Pressure Water Jetting (UHPWJ) equipment

• Water jetting use Rotating Nozzles
  • Nozzles can be tailor made to fit the substrate to be cleaned
  • Angle and capacity of the nozzles can be varied
• Manual water jetting gun:
  • Recoil force is quite strong:
  • Shoulder stock and hand grip can help to resist back force
  • It is important to rotate operators to avoid fatigue
• Robotic water jetting can be used on larger flat areas, such as on ships’ hull and storage tanks
  • The robotic equipment may operate suspended from a wire, from a mechanical arm or based on a vacuum-held crawler
  • Robotic UHPWJ equipment will usually collect the spent water and debris automatically and may have a processing unit to clean the water before recycling

UHPWJ Health & Safety

• The extreme high pressure of the water at the cleaning nozzle presents a safety hazard for the operator and observer
• Operators need to add special armoured PPE to their normal PPE when operating this equipment

Flash rusting

With UHPWJ leaving the surface very clean and wet, one might expect flash rust to occur quickly. The formation of flash rust is however often less than expected. The high-pressure water jet supplies energy to the substrate, resulting in heating of the surface. The rise of temperature results in quick evaporation of the water, which together with the lack of salts on the surface reduces the tendency for flash rusting to take place. A more prominent risk is posed by droplets flowing from later water jetting onto the already dry areas.

Flash rust

UHPWJ

Advantages

• Removes salt and rust in one step
• No dust produced
• No grit cost (water is cheaper in most areas)
• It is a noisy process, but has a lower noise level than grit-blasting
• Other trades can work nearby (in the neighbourhood)

Disadvantages

• Flash Rusting
• Capital cost of equipment
• Less efficient than abrasive blast cleaning
• No additional surface roughness (but will reveal existing anchor pattern)
• Equipment may be “bulky” for narrow spaces (ballast tanks)
• Water quality and availability
• Water freezes below 0°C
• Inhibitor may be necessary

Do “Rust Converters” work?

• Generally, you end up with a residue on the surface and that residue may itself be hygroscopic (water drawing) and may initiate osmotic blistering when overcoated
• Check with the coating manufacturer before allowing inhibitors to be used

There are several so called “rust converters” available:

• Phosphoric acid based (forms a layer of iron phosphate on the substrate)
• Tannin based (forms a layer of iron tannate on the substrate)
• Penetrating oils
• Systems transforming rust to Magnetite
• Systems incorporating corrosion inhibitors
• Proprietary “undeclared content” (other types)

The Australian Standard AS 2312.1 2015 states:

 “The use of a ‘rust converter’ with any of coating systems is not recommended, and will detract from their subsequent performance.”