ipcm Protective Coatings n. 37 - April 2021

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Coating Inspections: Inspection of Steel Surface Preparation, Part 2 Massimo Cornago NACE International Certified Coating Inspector, NACE CIP PEER Reviewer cornago@ipcm.it


s mentioned in our previous article1, we will now examine the most common surface preparation systems.

We will begin with the “water cleaning” method. This is often used in the preparation of both metal and cement surfaces, using quality-controlled industrial water that can be heated and used with the addition of a detergent, which optimises the cleaning action, or with abrasives, which can be injected into the water flow – in this case, the correct term is “water blasting”. Depending on the pressure used, water cleaning processes can be further defined as “power washing” or “water jetting” and classified according to SSPC-SP12/NACE 5 as follows:

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• low-pressure water cleaning, under 34 Mpa (5000 psi) • high-pressure water cleaning, in the range 34-70 Mpa (5000-10000 psi)

Equipment inspection

• high-pressure water jetting, in the range 70-170 Mpa (10000-25000

The equipment used for water cleaning is quite complex.


Its main components are as follows:

• ultrahigh-pressure water jetting, above 170 Mpa (25000 psi).

• A power source to operate the pump • A pump to pressurise water

Advantages and disadvantages of the two techniques

• A hose and fittings to delivery water to the nozzle

Although the water cleaning operation requires the addition of a

• A high-pressure nozzle

corrosion inhibitor to prevent flash rusting, it has the advantage

• A control valve system to enable to the operator to start and stop the

over conventional abrasive blasting of removing soluble salts as well.

water flow.

Environmentally acceptable sodium and potassium nitrite inhibitors are generally added.

If the job specification requires the water cleaning equipment to

Low-pressure water cleaning is effective in removing visible dirt and

be inspected, the manufacturer should be consulted for specific

mould on coated metal and it is generally safe for any adjacent wood,


concrete, and masonry (although it is always necessary to carry out

In each of the major components, some of the items that may require

a specific check first). High-pressure water jetting is generally able to

inspection are as follows:

remove loose rust and the majority of intact coatings: the very high-

• Power source and pump. It should be sufficient to maintain the

pressure jet can remove coatings and even rust. Water blasting cannot

required pressure during all the work phases. Only clean water should

be used to remove scale and roughen metal surfaces. Steel cleaned with

be used. Filters should be in place and working properly in order to

water will not be roughened, without any abrasive injection.

avoid clogging the system. • Hose and fittings. They should be of the right size and pressure rating and free of excessive wear. The fittings should be compatible with the

M. Cornago, Coating Inspections - Inspection of Steel Surface Preparation, Part 1, in ipcm Protective Coatings no. 36 (December 2020), pp. 40-41.




N.37 - 2021 APRIL - ipcm® Protective Coatings

hose and tested as a unit.