Blast Cleaning is a technique used in a broad range of industries, consisting of removing any dirt or scale from metal objects by an impinging action or an abrasive material propelled to its surface.
While this sounds very common and easy to achieve, an efficient blast cleaning involves a combination of aggressive and smooth actions that can quickly remove unwanted residues from the material without altering or modifying its surface.
Blast Cleaning is not to be confused with Surface Preparation and other blasting applications in which the surface can be modified, peened, formed, or compressed from its original structure.
Abrasive Cleaning or Blast Cleaning is the process of eliminating all visible dirt, rust, corrosion, carbon build-up, oxides, paint, functional coating, mill scale, dust, and other stubborn residues from the surface of an object by propelling abrasive media against it by any controlled means.
Wet contaminants, such as oil and grease, must be washed away with a solvent cleaner, and parts must be dried completely prior to using blast cleaning equipment. Alternatively, wet blasting can be used for such an application. Otherwise, these contaminants could cause coagulation of the abrasive and lead to machine failure in dry blasting applications.
Blast Cleaning is a very common application of sandblasting and can occur at various stages of the manufacturing, maintenance, or reconditioning of a metallic surface. Manufacturers and contractors use this technique on workpieces or structures made of metal, cast iron, plastic, concrete, composite, and other materials.
Blast Cleaning consists of propelling abrasive media at a high velocity – by the use of compressed air in sandblasting or wheel blast machines – onto the surface of a material to effectively dislodge unwanted residues from the surface or its cavities. In some circumstances, the media breaks upon impact (and becomes dust), but it could also remain intact and be reused later in the process with the use of a proper media reclaiming system. Blast equipment usually includes dust collectors which capture and filter dust at the source to avoid contaminating surrounding machines and the work environment.
Although the purpose of blast cleaning is not to modify the surface or the form of a workpiece, cleaning processes which are particularly aggressive or use angular, hard abrasive media sometimes result in the creation of spikes which are a particularity of Surface Preparation. Depending on the application, peaks caused by blast cleaning can be acceptable to achieve a faster cleaning process, or could lead to a rejection of the workpiece in processes with higher quality requirements.
Metallic surfaces are subjected to different sources of chemical contamination. The most prominent we can all think of is rust. However, while some other contaminants are not readily visible to the eye, they must still be removed in order to ensure the proper bonding of coating.
Rust and corrosion: Structural steelwork is heated at over 1,000ºC when fabricated. When it starts to cool down, the surface reacts very quickly to the ambient air (particularly oxygen) and we can start seeing the appearance of mill scale. Mill scale creates fissures, and when moisture present in ambient air starts to fill in, the rusting process of the structure occurs. With time, rust starts to flake, leaving the surface unstable for the proper bonding of coating. Even at an early stage of deterioration, mill scale could affect bonding and should be removed completely before painting.
Soluble salt: Other than corrosion, the presence of soluble salt on a structure could affect the bonding of coating. Old steel structures with signs of corrosion are more subject to having salts of ferrous sulfate and iron chlorides. In many cases, these salts must also be removed prior to applying a coating for maintenance jobs.
Wet contaminants: Machining or lubricating oil, grease, and other wet contaminants usually result from manufacturing processes. While such contaminants do not usually negatively affect the metallic structure – in fact, they usually act as a protective agent against corrosion and other chemical deterioration – they must be completely removed in order to ensure the proper bonding of coating.
The most common applications of blast cleaning are:
Construction and Civil Engineering
Surface preparation standards were jointly written by the technical committees of the Society for Protective Coatings (SSPC), NACE International Standard, and International Organization for Standardization (ISO).
Most standards define the cleanliness requirements of metals and concrete using a dry blast cleaning or a water jet cleaning process prior to applying protective coating. The cleanliness level refers to the amount of contaminants tolerated for a given surface area.
The most commonly used standards in blast cleaning are detailed in the tables below.
|Standards||Cleanliness Requirements||Surface Area Free of Contaminant|
|White Metal Blast Cleaning||When viewed without magnification, the surface must be free of all visible oil, grease, dust, dirt, mill scale, rust, coating, oxides, corrosion products, and other foreign matter (White Metal Requirements).||100%|
|Near-White Metal Blast Cleaning ||White Metal Requirements applied on at least 95% of each unit area. Staining will be limited to no more than 5% of each unit area, and may consist of light shadows, slight streaks, or minor discolourations caused by stains of rust, mill scale, or previously applied coatings. Unit area shall be approximately 3 in. x 3 in. (9 sq. in.).||95%|
|Industrial Blast Cleaning ||Removal of all visible oil, grease, dust, and dirt when viewed without magnification. Traces of tightly adherent mill scale, rust, and coating residues are permitted to remain on 10% of each unit area of the surface if they are evenly distributed. Shadows, streaks, and discolouration caused by stains of rust, mill scale, and previously applied coating may be present on the rest of the surface.||90%|
|Commercial Blast Cleaning||White Metal Requirements applied on at least two thirds (2/3) of the unit area, which must be a square 3 in. x 3 in. (9 sq. in.). Light shadows, slight streaks, or minor discolourations caused by stains of rust, mill scale, or previously applied coating in less than 33-1/3% of the unit area are acceptable.||66%|
* Joint Standards developed jointly by all three copyright holders
Some standards refer to cleaning processes other than abrasive blast cleaning. While blast cleaning is usually more efficient than other techniques to rip off scale, stains, and rust onto a workpiece, solvent cleaning is usually preferred to remove chemicals, grease, and soluble salts on metal surfaces, as these contaminants could affect the functioning of sandblasting equipment. A combination of various methods can be used to achieve the level of cleanliness required for a given application.
|Solvent Cleaning ||Removes all visible oil, grease, soil, drawing, and cutting compounds, as well as other soluble contaminants from steel surfaces with solvent, vapour, cleaning compounds, alkali, emulsifying agent, or stream.|
|Hand Tool Cleaning ||Removes all loose mill scale, rust, paint, and other detrimental foreign matter by hand tool chipping, scraping, sanding, and wire brushing.|
|Power Tool Cleaning ||Removes all loose mill scale, rust, paint, and other detrimental foreign matter by power tool chipping, descaling, wire brushing, sanding, and grinding.|
|Brush-Off Blast Cleaning ||When viewed without magnification, the surface must be free of all visible oil, grease, dirt, and dust, as well as loose mill scale, rust, and coating. Tightly adherent mill scale, rust, and coating may remain on the surface. Mill scale, rust, and coating are considered tightly adherent if they cannot be removed by lifting with a dull putty knife.|
|Power Tool Cleaning for Bare Metal ||When viewed without magnification, the surface must be free of all visible oil, grease, dirt, dust, mill scale, rust, paint, oxides, corrosion products, and other foreign matter. Slight residues of rust and paint may be left in the lower portion of pits if the original surface is pitted. The surface profile must not be less than 1 mil (25 microns).|
|Surface Preparation and Cleaning of Steel and Other Hard Materials by High- and Ultra High- Pressure Water Jetting Prior to Recoating ||This standard requires water jetting at high- or ultra-high-pressure to prepare a surface for re-coating using pressure above 10,000 psi. Water jetting will not produce a profile, but rather expose the original abrasive-blasted surface profile. Water jetting shall be performed to meet four conditions: WJ-1, WJ-2, WJ-3, and WJ-4, and a minimum acceptable surface must have all loose rust, mill scale, and coatings uniformly removed.|
|ISO 8501 Visual assessment of surface cleanliness|
|ISO 8501-1:2007 Part 1: Rust grades and preparation grades of uncoated steel substrates and steel substrates after overall removal of previous coatings.|
|ISO 8501-2:1994 Part 2: Preparation grades of previously coated steel substrates after localized removal of previous coatings.|
|ISO 8501-3:2006 Part 3: Preparation grades of welds, edges, and other areas with surface imperfections.|
|ISO 8501-4:2006 Part 4: Initial surface conditions, preparation grades, and flash rust grades in connection with high-pressure water jetting.|
|ISO 8502 Tests for the assessment of surface cleanliness|
|ISO 8502-2:2017 Part 2: Laboratory determination of chloride on cleaned surfaces.|
|ISO 8502-3:2017 Part 3: Assessment of dust on steel surfaces prepared for painting (pressure-sensitive tape method).|
|ISO 8502-4:2017 Part 4: Guidance on the estimation of the probability of condensation prior to paint application.|
|ISO 8502-5:1998 Part 5: Measurement of chloride on steel surfaces prepared for painting (ion detection tube method).|
|ISO 8502-6:2006 Part 6: Extraction of soluble contaminants for analysis – The Bresle method.|
|ISO 8502-9:1998 Part 9: Field method for the conductometric determination of water-soluble salts.|
|ISO 8502-11:2006 Part 11: Field method for the turbidimetric determination of water-soluble sulfate.|
|ISO 8503 Surface roughness characteristics of blast-cleaned steel substrates|
|ISO 8503-1:2012 Part 1: Specifications and definitions for ISO surface profile comparators for the assessment of abrasive blast-cleaned surfaces.|
|ISO 8503-2:2012 Part 2: Method for the grading of surface profile of abrasive blast-cleaned steel – Comparator procedure.|
|ISO 8503-3:2012 Part 3: Method for the calibration of ISO surface profile comparators and the determination of surface profile – Focusing microscope procedure.|
|ISO 8503-4:2012 Part 4: Method for the calibration of ISO surface profile comparators and the determination of surface profile – Stylus instrument procedure.|
|ISO 8503-5:2017 Part 5: Replica tape method for the determination of the surface profile.|
|ISO 8504 Surface preparation methods|
|ISO 8504-1:2000 Part 1: General principles.|
|ISO 8504-2:2000 Part 2: Abrasive blast-cleaning.|
|ISO 8504-3:1993 Part 3: Hand- and power-tool cleaning.|
Coating and lining specifiers, quality control managers, applicators, and inspectors use these standards to define whether or not a blast cleaning process has achieved its objective or process requirements.
Achieving high-quality blast cleaning 100%-free of contaminants is not always required. In fact, it can be expensive and requires fine-tuned equipment with the full process controlled. Painting and coating manufacturers usually specify the minimum standard that is required for their coating products to adhere properly. When a high-performance re-coating is not required, SSPC-SP14 / NACE 8 Industrial or SSPC-SP6 / NACE 3 Commercial Blast Cleaning usually apply. Make sure to follow your painting or coating provider’s requirements prior to applying coating – a cleaning process that does not meet the minimum requirements may lead to premature paint failure.
Keep in mind that once the cleaning process is over, parts need to be dried completely and a protective coating should be applied immediately. Otherwise, blasted surface exposed to salts, moisture, contamination, or a corrosive environment can rust very quickly.
Typically, two measuring tests can be used to assess the quality of the blast cleaning application: the chloride test and the comparison chart.
It is extremely important to remove any chloride salt remains on the surface of a steel surface prior to applying coating. Soluble Salts – such as chloride, sulphate, or nitrate ions – can draw moisture through the paint and lead to premature coating failures. Eventually, the corrosion process will occur on certain areas of the unclean steel surface if soluble salt remains are found in moderately high concentrations on a steel surface. Unfortunately, abrasive blasting does not remove soluble salt contamination. Even worse, cross contamination could happen between jobs when reusing abrasive media for different applications.
The most common method used to assess the presence of soluble salts on a metal surface is the Bresle Patch Chloride Test. This method has been used for over 20 years to measure the concentration of all soluble salts on steel surfaces in accordance with ISO 8502-6 and ISO 8502-9.
The Bresle Patch Chloride Test consists of measuring the concentration of soluble salts on a sample area of the steel surface by using a patch, a syringe, a small amount of distilled water, and a measuring apparatus.
Step 1 – Stick a patch compartment on a sample area of the steel surface.
Step 2 – Add a small quantity of water (3ml) and remove air from the patch compartment.
Step 3 – Massage the water compartment to dissolve all soluble salts into the mixture and let stand for 10 minutes. During that time, a repeated series of water removal and reinsertion is recommended to make sure the water collects all soluble salts.
Step 4 – Collect water from the patch compartment and add a few drops in the measuring apparatus to measure the electric conductivity of the mixture (since salted water is highly conductive, the level of conductivity is correlated to obtain the concentration of soluble salts).
Another widely used method to measure the cleanliness of a metal surface is the comparison chart – also referred to as the magnifying glass or the visual chart method. This method consists of comparing the treated metal surface with reference photographs.
The ISO 8501 is a pictorial standard showing different rust grades and levels of cleanliness, though it also contains text descriptions of the cleanliness levels. SSPC/NACE also sell visual guides for direct comparison with surfaces.
The method is very straightforward. As the title suggests, job coaters must simply compare the metal surface with a reference image contained in the standard. The use of a lighted magnifying glass is required in order to achieve consistent and repeatable comparisons. The result of this method can vary from one inspector to another and is open to interpretation, but it is the best method found so far.
In blast cleaning, abrasive media with low hardness levels are usually used in order to avoid stripping or chipping the workpiece. For precision blasting, very small media grade (MESH) is usually used – while larger-sized particles tend to be more efficient in terms of production rate, these can also lead to damaging the surface blasted. When it comes to choosing the right abrasive for your application, keep in mind the composition of material, the surface condition, and the types of contaminant found on the surface.
Ceramic Beads and Glass Beads are the most commonly used abrasive media for blast cleaning on metal surfaces with medium-to-low contaminant levels. These abrasives have the ability to efficiently remove contaminants from the surface while preserving the base material with minimal damage and contamination. The spherical shape of these abrasives is softer on impact than angular media, and they usually break into dust upon impact – which is a decent price to pay to avoid damaging the surface of the workpiece.
For more aggressive blast cleaning, hard minerals – such as GMA Garnet and JetMag – can be used to effectively and rapidly rip off contaminants from surfaces. These medias have an angular shape, which leads to a more aggressive action upon impact. However, they can also strip off or damage the surface when used on inappropriate material.
Cast iron or very hard metal alloys can be cleaned with steel media such as Steel Shot or Steel Grit. In addition to being very efficient in cleaning a surface, steel media also have the ability to resist impact and keep their shape for a long life in a closed-cycle system. On the other hand, they also require a powerful system to be used appropriately (vacuum systems are usually not powerful enough to deal with such heavy abrasive media).
Aluminum Oxide, another angular-type media, is very good for light deburring. It also has a great recyclability since it can be reused 10-12 times through the blast system.
For blast-cleaning delicate objects such as plastic parts and molds, dies, electronic connections, and circuit boards, Plastic Media with a spherical or angular shape is the best option to keep objects intact. Note that plastic media are usually used in relatively low-pressure processes (30-50 psi).
Organic media, such as Walnut Shell, can be used on alloys that cannot be contaminated with steel particles which can cause rust, for stripping the first layer of coating on submarines and aircraft without damaging expensive undercoats, and for cleaning turbines and other delicate parts on engines. Organic media is also environmentally friendly, biodegradable, and safe for health.
Refer to the IST Media Selection Guide in order to choose the right media for your application.
There are several types of blast equipment that could be used, depending on the size of the parts to be cleaned. Blast pots are an efficient form of equipment with a very small footprint that could be used with pretty much any type of abrasive media directly on the job-site of a blasting facility. Larger equipment such as blast rooms can also provide a controlled environment for industrial shot blasting. Automated blasting machines can also be used to achieve consistent results and productivity. This equipment is usually powered by a proper air compressor that can supply enough velocity (CFM) at the exit end of the nozzle. The use of safety equipment such as respiratory masks and blast suits is strongly recommended in order to protect the worker from hazardous flying particles.