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How Steel Grit Blasting Can Improve Productivity and Quality

TUESDAY, APRIL 24, 2018

By Mark Stewart, Gus Lyras, Nichole DeSantis, ARS Recycling Systems, LLC


Abrasive blasting requires propelling media onto a steel surface to prepare it for coating. Mineral abrasives — such as silica sand, coal slag, garnet sand, nickel slag and copper slag — are blasted, turned to dust upon impact and disposed of. Unlike these and other natural mineral abrasives, steel grit can be recycled more than 100 times before it goes to waste.

The recycling process assists in removing hazardous materials that could lead to environmental problems or premature paint failure. When the proper process is followed, steel grit can be cleaned to near-new condition between reuses and provide uniform surface preparation. The contractor can then ensure higher quality, as the process can decrease risks associated with coating failures and increase disposal savings, logistics savings, productivity, and benefits to health and the environment.

THE THREAT OF CONTAMINANTS

Structures such as bridges, tanks and ships typically host many contaminants. When blasted onto the painted steel during surface preparation, abrasive media picks up these contaminants as well as corrosion from the structure. If reused without proper cleaning, the media could introduce those contaminants onto the next surface and into the environment.Two primary contaminants are lead and chlorides.

Lead

For decades, lead-based paint was chosen to coat highway bridges because it is highly effective as a corrosion inhibitor; however, lead-containing paints have proven to be toxic to people and the environment.2 With about 70 percent of bridges still covered in lead-based paint, abrasive blasting workers are at high risk for lead exposure, prompting OSHA to establish many regulations and standards to help reduce this exposure.3 SSPC (the Society for Protective Coatings) created a standard, SSPC-AB2, that includes a requirement for cleanliness of steel grit — less than 0.2 percent lead content.4

Chloride

Chlorides and other soluble salts are the primary cause of paint failure and structure deterioration.2 Chlorides become present on structures due to salt contaminants in the atmosphere and in applications for snow and ice removal.5 Other sources of chloride contaminants are found on abrasive material.6 If the contaminants are not removed before blasting, there may be loss of adhesion, blistering and corrosion beneath a coated surface, resulting in coating failure.7

KEEPING IT CLEAN

Steel grit is manufactured in a controlled environment; thus, it contains fewer contaminants than other abrasives — including natural minerals — before and after being blasted. The microstructure of high-carbon cast-steel grit consists of tempered martensite, which holds a certain hardness range. The harder the grit, the more friable it becomes, which means grit will hold its sharp edges and remain an aggressive blasting media for multiple uses.8 Due to its molecular compound, steel grit can be repeatedly blasted, recycled and cleaned, and added back into the work mix for a uniform blast profile. As steel grit breaks down to form a balanced  operating mix, its performance is analyzed by NACE and SSPC cleanliness standards.

Surface preparation accounts for 40 percent of the cost associated with a repainting job; thus, contractors and facility owners need to carefully analyze the material they are using to blast.9

Contractors can enjoy benefits that do not exist when blasting with mineral aggregate. These include a 99-percent recyclability rate and non-fracturing, dustless media properties in addition to the cost savings, quality, productivity and health benefits noted earlier.

SAVING MATERIAL, MONEY AND THE ENVIRONMENT

The cost savings associated with using steel grit are predominantly derived from the recyclability rate that steel grit can uphold. Steel grit would not maintain its quality and cleanliness after being blasted without going through a proper recycling and cleaning process. With the right technology, steel grit will be thoroughly cleaned using a four-stage recycling process, beginning at stage one in the recycler — the baghouse collection — and ending at stage four — the multi-stage air-wash station.

Some recycling machines stop the cleaning process at cyclonic separation, which keeps the steel grit from meeting SSPC-AB2 and may reintroduce contaminants onto the blasted surface. Laboratory test results show that, by using proper technology with the more thorough four-stage process for recycling and cleaning dirty grit, the abrasive can meet all required cleanliness standards, including SSPC-AB2. The tests described below not only demonstrate that lead levels are acceptable after recycling and cleaning but that chloride levels are also decreased.

Test 1

Two samples of steel grit were tested in a certified laboratory to measure the total lead and chloride content. The results compared the amount of contaminants in new steel grit (aka clean grit) to the amount in grit blasted onto a steel surface with 10 to 15 mils of lead-based paint, then picked up off the contaminant floor (aka dirty grit).

In Test 1, dirty grit had signifcantly higher lead levels than the clean grit. The dirty grit had a total lead content of .638 percent - three times the allowable amount of contaminants in SSPC-AB2 - yet many contractors continue to blast with dirty grit because they do not have a proper recycling and cleaning system.

Initially, clean grit had 15 mg/kg (0.0015 percent) of choloride, but after it was blasted on a steel structure, the sample contained 28.8 mg/kg (0.00288 percent) of chloride.

Test 2

The dirty grit sample, examined in Test 1, was put through the four-stage cleaning process, beginning with a baghouse collection and ending in a multi-stage air-wash station. The recycled grit was then tested in a certified laboratory for total lead and chloride content.

The lead and chloride levels found in the recycled grit were far lower than those found in the dirty grit. The cleaning process had decreased lead levels by 5,240 mg/kg, or 82 percent, and  the resulting 0.114-percent lead level surpassed the SSPC-AB2 standard. This means that the specific recycling process used to clean the dirty grit in Test 2 truly cleaned it. The process also cleaned chloride ions off the steel grit, which provides a cleaner finished blast surface.

With regard to health, properly removing contaminants from steel grit — and reusing it — minimizes the amount of hazardous waste sent for disposal, making it far better for the environment than using non-recyclable or poorly cleaned abrasives.

Test 3

Three steel plates, each approximately 3 by 3 inches, with mill scale present, were solvent-cleaned and then blasted with new grit, dirty grit and cleaned/recycled grit, respectively, at a psi of 90. Ghost wipes were then used to wipe each plate to test the lead levels each steel grit sample imbedded into the steel plate. The ghost wipes were then sealed and brought to a certified testing laboratory. Below are the results.

New, "virgin" grit initially has a small amount of lead and chloride in the media, but once blasted on a steel surface, the abrasive becomes highly contaminated. Test 3 results indicate that properly recycled steel grit is cleaned well enough to blast at quality levels similar to virgin grit, thereby keeping contaminants from being reintroduced into the blasted steel. Not only does this reduce the amount of hazardous pollutants released into the air, but it yields cost-savings to both contractor and owner.

Below is an overview of the three samples tested, demonstrating the amount of lead and chloride in the abrasive.

Conclusion

As detailed in “The Benefits of Steel Grit Blasting and Recycling,” a contractor can save up to 81 percent of abrasive, disposal and labor costs by using recyclable steel grit. This article has provided quantitative data on the benefits of steel grit cleaning and recycling. With a four-stage cleaning/recycling process, steel grit cleans contaminants that may have, otherwise, caused damage to the environment and premature paint failure. In the long run, the continuous removal of contaminants, such as lead and chloride, throughout the cleaning/recycling process, improves productivity and quality of the surface profile, which allows for a higher cost savings.

References

  1. Hudson, R. "Guides to Good Practice in Corrosion Control: Surface Preparation for Coating." National Physical Laboratory, 2000.
  2. Baboian, Robert. Corrosion tests and standards: application and interpretation. 2nd ed., ASTM International, 2006.
  3. "Lead in Construction." U.S. Department of Labor: Occupational Safety and Health Administration, 2004.
  4. "Standard: SSPC AB 2." SSPC AB 2 - Cleanliness of Recycled Ferrous Metallic Abrasives. Engineering360.
  5. Marrow, David D, and Wallace P Cathcart. "Chloride Contamination on Abrasive." PaintSquare, Jan. 1992.
  6. Maupin, G. W., and Abba G. Lichtenstein. Extending the Life of Bridges. ASTM, 1990.
  7. "Surface Preparation Standards Explained." Blast Journal, 2017.
  8. "Steel grit and shot: Spherical steel blasting shot." Steel grit and shot, Fornitech, 2017.
  9. "Soluble Salts." Soluble Salts and Their Impact on Corrosion Control. CHLOR RID International, Inc., 2017.

*Claims or positions expressed by sponsoring authors do not necessarily reflect the views of TPC, PaintSquare or its editors.

ABOUT THE AUTHORS
Mark Stewart, Gus Lyras, Nichole DeSantis, ARS Recycling Systems, LLC
     

Nichole DeSantis (left), market analyst, obtained her B.S. in marketing and is currently finishing her MBA. She is employed full time at ARS in the sales department. Gus Lyras (center) is co-founder and vice president of ARS Recycling Systems. Mark Stewart (right) obtained his B.S. in electrical engineering and an MBA in technology. He has grown manufacturing organizations and is now president of ARS.

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Tagged categories: Blasting; Steel Grit; Surface preparation; Surface Preparation

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