Waterjet Technologies

Water as blasting media.

Hydroblasting, water jetting or water blasting are terms commonly used when describing water as the tool to do surface preparation. The use of water is a technique for cleaning surfaces, which relies solely on the energy and force of water striking a surface to acheive its cleaning effect. In principel abrasive is not used in hydroblasting systems, hence, the problems caused by dust pollution and by the disposal of spent abrasives are eliminated. However wet-sand blasting is often used in conjuction with 500 to 700 bar water blasters to enhance the productivity and surface finish.  

Surface Condition after Hydroblasting.


Flash rust:

When large areas are hydroblasted, flash rusting which deminish the original surface standard may occur, before painting can be carried out. The degree of flash rusting depends on many factors, such as humidity level, temperature and the length of time the surface is exposed.

Degrees of flash rust:

- Light flash rust     : small quantities of light tan-brown rust will partially discolour the original steel surface. The discolouration may be evenly distributed, or in patches, but it will not be heavy enough to mark an object brushed against the areas.

- Medium flash rust : a layer of light tan-brown rust will cover the original steel surface. This layer may be evenly distributed or in patches, but it will be heavy enough to easily mark objects brushed against the areas.

- Heavy flash rust   : a heavy layer of dark tan-brown rust will be completely covering the original surface. This layer will be evenly distributed and will easily mark objects brushed against the areas.  

Is the flash rust too heavy for coating application, it should be removed by washing down with high pressure fresh water, ideally with a fan jet nozzle using the same equipment as used for hydroblasting at lower pressure. It will cause the surface to re-rust, but it is possible to reduce the degree of flash rusting from heavy to light.

All paint manufacturer have "modified epoxy" paint that can be applied directly on areas with light rust without compromising the lifespand of the coating.


Temperature rise:

The use of hydroblasting will cause the temperature of the steel to rise. The increase of temperature of the steel surface can be substantial, depending on the pressure of the equipment used and will help hydroblasted surfaces to dry off faster, hence reducing the degree of flash rusting.


Removal of foreign elements :

The use of water has the advantage of removing oil and grease from the surface along with salt deposits. This is a major plus point of hydroblasting. The grey, brown to black discoloration of corroded and pitted steel after hydroblasting cannot be removed by further blasting. Studies shows that this thin film consists of mainly ferric oxide, which is an inert material. As it is tightly adherent, it does not present a serious contamination problem.

Surface Profile.

Hydroblasting will not produce a surface profile as such. Weakend steel might erode, resulting in metal loss, but the surface profile exposed after blasting will have been produced by earlier surface preparation or corrosion.

Surface Preparation Standards.  

Surface preparation standards are stipulated by various bodies throughout the world to designate the cleanliness condition of blasted steel, prior to applying a protective coating.

The applicable cleanliness standard is usually called for by either the protective coating manufacturer or the owner of the structure to be painted.

The most commonly referred to standards are SSPC, NACE, Swedish Standards and ISO.

Each standard is divided into four standards of cleanliness, broadly described as follows; brush off, commercial, near white metal, white metal. Whilst each standard may differ slightly in requirements and terminology the following cross reference chart indicates the close approximation of each level of cleanliness for each standard.

                       BRUSH OFF         COMMERCIAL           NEAR WHITE METAL            WHITE METAL

SSPC              SP 7 - WJ 4          SP 6 - WJ 3                SP 10 - WJ 2                     SP 5 - WJ 1 

NACE                  No.4                     No.3                           No.2                                No.1

SWEDISH            Sa.1                     Sa.2                           Sa.2.5                             Sa.3

ISO 8501             WA.1                    WA.2                          WA.2.5

International        HB.1                     HB.2                           HB.2.5

A brief description of each of the four levels of cleanliness is as follows:

Brush Off - Loose mill scale, loose rust and foreign particles are removed.

Commercial - Mill scale, rust and foreign particles are substantially removed and grey metal is visible.

Near White Metal - Mill scale, rust and foreign particles are removed to the extent that only traces remain in the form of spots or stripes. The cleaned surface will show varying shades of grey.

White Metal - Visible mill scale, rust and foreign particles are entirely removed. The cleaned surface should have a uniform metallic colour but may show varying shades of grey when viewed at different angles.


The full standards are available from each organisation from the following links:

National Association of Corrosion Engineers     http://www.nace.org/

Steel Structures Painting Council                    http://www.sspc.org/

International Organization for Standarization   http://www.iso.ch/

Swedish Institute for Standards                      http://www.sis.se/


The appearance of steel after blasting will vary greatly and is subject to, amongst other things, the condition and appearance of the steel prior to blasting. Other factors that influence the post blasting appearance include, angle of blast and profile height.