Waterjet and wet abrasive blast cleaning methods

Waterjet and wet abrasive blast cleaning methods

Waterjet and wet abrasive blast cleaning are recognized methods for removing coatings and other contaminants to prepare existing coated structures for protective coating application.

These methods have evolved over the years to meet various challenges and service environments in the field. As the techniques and equipment have matured, so have the applicable standards and training surrounding them, which are explained in greater details below.

Wet abrasive blasting: A dual-component approach

Wet abrasive blast cleaning (WAB) is a generic description of systems that clean, prepare, or cut surfaces by delivering water, compressed air, and abrasive material. In some systems, water and air are the primary actors with abrasives added in. In others, abrasives and air are the primary actors with water added in.

WAB systems go by many names, including water-shroud blasting, wet-head blasting, water blasting, vapor blasting, and slurry blasting. Their main differentiator from dry abrasive blasting is that significantly less dust is created. Instead, the added water binds to fine particles and weighs them down, preventing their dispersion. That makes these systems viable when working in close quarters, when ventilation is poor, or when removing coatings that contain toxic materials.

Surfaces should be power washed following WAB cleaning to clear them of any leftover abrasive materials. Protective coatings bond best to clean surfaces. This is especially important for metal surfaces because, in addition to poor adhesion, leftover materials contribute to flash rust formation.

To ensure quality, consult the following SSPC/NACE WAB surface cleanliness standards for carbon steel surfaces:

  • SP 5 – White Metal Wet Blast Cleaning
  • SP 10 – Near White Metal Wet Blast Cleaning
  • SP 6 – Commercial Wet Blast Cleaning
  • SP 7 – Brush-off Wet Blast Cleaning
  • SP 14 – Industrial Wet Blast Cleaning

AMPP is home to various SSPC reports that address procedures, equipment, materials, and safety concerns pertaining to WAB systems.

Waterjet cleaning: It’s just water

Waterjet cleaning (WJ) systems only use water in conjunction with compressed air. They’re commonly used in maintenance or refurbishment settings to remove coatings, rust, or other materials, exposing the original substrate.

Recommended waterjet cleaning methods vary depending on the surface cleanliness specified. SSPC/NACE’s surface cleanliness standards for metal surfaces are:

  • WJ-1 - Waterjetting to Bare Substrate.
  • WJ-2 - Very Thorough Waterjetting.
  • WJ-3 - Thorough Waterjetting.
  • WJ-4 - Light Waterjetting.

Waterjet cleaning methods range in intensity, from relatively tame water cleaning to ultra-high pressure waterjetting. Here’s a rundown:

  • Water cleaning (WC) uses water under very little pressure to rinse unwanted matter from a surface.
  • Low-pressure water cleaning (LP WC) is carried out at pressures below 5,000 psig. Also called “pressure washing” or “power washing,” this method is used to remove loose materials like laitance, dirt, scale, or light marine growth from a surface.
  • High-pressure water cleaning (HP WC) features pressures between 5,000 and 10,000 psig. The method is suited for clearing pipe or removing weak concrete, marine growth, sandstone, mudstone, loose scale, loose rust, paint, and product accumulation on floors at lower pressures. At higher pressures, HP WC can remove concrete in piping, severe marine fouling, rubber on runways, lime scale, burned oil deposits, petrochemicals, stains, vegetation, tar, cement, and asphalt on vehicles or pipe. It can also restore building facades and expose aggregate.
  • Waterjetting (WJ) features surface preparation water delivered to a surface at pressures greater than 10,000 psig ahead of an inspection or coating application.
  • High-pressure waterjetting (HP WJ) is carried out at pressures up to 30,000 psig and is suited for paint or mill scale removal, removal of burned deposits and for cutting concrete.
  • Ultra-high pressure waterjetting (UHP WJ) is performed at pressures greater than 30,000 psig. It’s used mainly to remove non-skid coatings from ship flight decks but can also remove paint and mill scale. In addition, it can scour a surface ahead of dry abrasive blasting.

It’s important to note that although waterjetting can effectively clean a steel surface, it will not create a surface profile or anchor pattern that is essential for good coating adhesion. However, by removing existing coatings and contaminants, the existing surface profile will be exposed.

Flash rust considerations

Any time WAB or WJ cleaning is performed, flash rust may develop as non-stainless-steel surfaces dry. Flash rust can be classified into three levels:

  • Light flash rust is characterized as small amounts of rust in a thin layer through which the underlying substrate is visible. It cannot be easily removed when a cloth is wiped over the surface.
  • Moderate flash rust obscures the steel surface when viewed without magnification. It is well-adherent but will leave marks on a cloth wiped over the surface.
  • Heavy flash rust completely hides the surface from view. It’s loosely adherent and leaves significant markings on a cloth wiped over the surface.

To learn more about how to test for flash rust, consult the National Shipbuilding Research Program’s flash rust field assessment guidelines.