Steel Surface Preparation by Hydroblasting 109 i Figure 4.22 Special body protection for hydroblasting operators photograph: Warwick Mills, New Ipswich.. fresh water requirements, the c
Trang 1108 Hydroblasting and Coating of Steel Structures
(a) Wet suit, gloves, boots (b) Helmet with face and hearing protection
Figure 4.21 Protective clothing for hydroblasting operators (photographs: WOMA GmbH, Duisburg)
hazards in relation to the work being undertaken (see Fig 4.2 l(a)) This must
be used where there is a risk to health or a risk of injury
Hand protection (rubber gloves, reinforced gloves): Hand protection shall be supplied to all team members and shall be worn where there is a risk of injury
or contamination to the hands (see Fig 4.21(a))
Foot protection (steel-toed boots): All operators shall be supplied with suitable boots or Wellingtons with steel toe caps, and where necessary additional strap-on protective shields (see Figs 4.21(a) and 4.22)
0
0
These shall be worn when there is a risk of injury
0 Respiratory protection (sometimes with supplied air): see Section 4.4.2.3): Where necessary, suitable respiratory protection which is either type approved
or conforms to an approved standard must be worn
Typical personnel protective clothing and equipment for hydroblasting operators are shown in Figs 4.21 and 4.22 Table 4.19 lists results of direct water jet impact tests on the body protection worn by the operator in Fig 4.22 Further recommendations are given by French (1998), Momber (1993a), Smith (2001) and Vijay (1998b)
The use of hydroblasting equipment for the surface preparation on ships on sea, which often includes ballast tank cleaning, requires special safety and health considerations to establish the following parameters (Henderson, 1998):
0 where best to place the units on deck?
the best method of securing the units?
Trang 2Steel Surface Preparation by Hydroblasting 109
i
Figure 4.22 Special body protection for hydroblasting operators (photograph: Warwick Mills, New Ipswich)
Table 4.19 Results of resistance tests with body protection (Anonymous, 2002a)
~~~
Operating Volumetric Nozzle Distance Traverse Exposure Result pressure flow rate diameter i n m speed time'
~ ~~~
lCalculated with dNIvT
0 optimum hose runs:
0
0 ventilation trunking requirements:
the capacity, number, and type of ventilation fans required:
the ship's power supplies, their location, voltage, amperage, and cycles:
Trang 3110 Hydroblasting and Coating of Steel Structures
0
0
0
0
0 accommodations arrangements for hydroblasters
fresh water requirements, the capability of the vessel to supply sufficient fresh water for the work and the location of the supply points:
entry and exit points in each tank for personnel and equipment;
requirements for access equipments in the tanks:
lightning requirements and how to best illuminate substrates:
4.4.5 Confined Spaces
Surface preparation jobs as well as painting jobs are often performed in confined spaces, for example, manholes, pipelines, storage vessels, bridge box beams, interior
tower cells and ballast tanks A typical example is shown in Fig 4.21 Not all con-
fined spaces are considered hazardous However, they must be considered hazardous
if they contain or have the potential to contain the following (OSHA, 1993):
Hazardous atmospheres
This includes (i) lack of oxygen, (ii) presence of explosive gases and vapours and (iii) presence of toxic dusts, mist and vapours
Engulfment hazards
This includes spaces containing materials like salt, coal, grain and dirt that can easily shift and trap an operator
An internal configuration (slopes or inward configurations) that could trap or
asphyxiate
This includes spaces where the bottoms are sloped or curved (e.g narrow openings at the bottom of a silo) may trap or asphyxiate operators
Any other recognised serious hazards
This includes moving parts, power connections, liquid and anything else that can cause bodily harm
This special situation requires special training because it is reported that operators are still getting hurt in confined spaces The most important things to understand about hazards in confined spaces are as follows (Platek 2002):
0
0
0
0
0
What hazard will be encountered?
What equipment or means will offer protection from those hazards?
How the equipment is used?
Who can perform the work?
What happens if something goes wrong?
When a confined space is evaluated, three questions regarding that space should be answered
0
0
Is the space large enough that the operator can place part or all of his body
into it?
Does it have limited entry and exits?
Is it designed to work in continuously?
Trang 4S k 1 Surjace Preparation Hydroblasting 1 11
Training and education are the major methods to reduce risks if work is performed
in confined spaces OSHRA 29 CFR 1910.146 states: ‘The employer shall provide training so that all employees whose work is regulated by this section acquire the
understanding, knowledge, and skills necessary for the safe performance of the duties assigned under this section.’ Adequate training must be delivered when permit-required confined spaces are encountered and for all of the duties performed
in and around a confined space
Trang 6CHAPTER 5
Surface Quality Aspects
5.1
5.2
5.3
5.4
5.5
5.6
5 7
5.8
Surface Quality Features
Adhesion Strength
5.2.1 Definitions and Measurement
5.2.2 Adhesion to Bare Steel Substrates
5.2.3 Integrity of Remaining Coatings
Flash Rust
5.3.1 Definitions and Measurement
5.3.2 Effects on Coating Performance
Non-Visible Contaminants - Salt Content
5.4.1 Definitions and Measurement
5.4.2 Effects on Coating Performance
5.4.3 Substrate Cleanliness after Surface Preparation
Embedded Abrasive Particles
5.5.1 General Problem and Particle Estimation
5.5.2 Quantification and Influence on Coating Performance
Wettability of Steel Substrates
Roughness and Profile of Substrates
5.7.1 Influence of Roughness on Coating Adhesion
5.7.2 Influence of Roughness on Paint Consumption
5.7.3 Surface Profiles on Remaining Coatings
5.7.4 Profiles on Hydroblasted Steel Substrates
5.7.5 Profiles on ‘Overblasted’ Steel Substrates
Aspects of Substrate Surface Integrity
Trang 7114 Hydroblasting and Coating of Steel Structures
5.1 Surface Quality Features
IS0 8502 (1995) states the following: ‘The performance of protective coatings of
paint and related products applied to steel is significantly affected by the state of the steel surface immediately prior to painting The principal factors to influence this performance are:
(i)
(ii)
(iii) the surface profile.’
the presence of rust and mill scale:
the presence of surface contaminants, including salts, dust, oil and greases:
Numerous standards have been issued to define these factors (see also Chapter 6),
and testing methods are available to quantify them Hydroblasted surfaces show some distinct features, and extensive experimental studies have been performed to address this special point, often in direct comparison to other surface preparation methods
5.2 Adhesion Strength
5.2.1 Definitions and Measurement
According to Bullett and Prosser (1972) ‘the ability to adhere to the substrate throughout the desired life of the coatings is one of the basic requirements of a sur- face coating, second only to the initial need to wet the substrate.’ Adhesion is based upon adhesive forces that operate across the interface between substrate and applied coating to hold the paint f l to the substrate These forces are set up as the paint is applied to the substrate, wets it, and dries The magnitude of these forces (thus, the adhesion strength) depends on the nature of the surface and the binder of the coating Five potential mechanisms cause adhesion between the surfaces of two materials:
0 physical adsorption;
0 chemical bonding:
0 electrostatic forces:
0 diffusion:
0 mechanical interlocking
In the mechanical interlocking mechanism, the macroscopic substrate roughness provides mechanical locking and a large surface area for bonding; the paint is mechanically linked with the substrate Adhesive bonding forces could be cate- gorised as primary valency forces and secondary valency forces as listed in Table 5.1 Adhesion depends on numerous circumstances, among them substrate profile (see Section 5.7), substrate cleanliness (see Section 5.3) and type and application of the subsequent coating system Adhesion between substrate and coating can be
Trang 8Table 5.1 Bonding forces and binding energies (Hare, 1995)
in kcalhole
~~
Ionic
Covalent
Coordinate
Metallic
Hydrogen
bonding
~ ~ Primary valency
Primary valency Primary valency Primary valency
Secondary valency
Dispersion Secondary valency
Dipole Secondary valency
Induction Secondary valency
Most organic molecules Quaternary ammonium Bulk metals
compounds
Water
~ Bonding formed by transfer of valency electrons from Metal salts the outer shell of a n electron-donating atom
into outer shell of an electron-accepting atom to produce a stable valency configuration in both
Bonding formed when one or more pairs of valency electrons are shared between two atoms
Covalent type bond where both the shared pair of electrons are derived from one of the two atoms
Bonding in bulk phase of metals between positively charged metallic ions and the electron
cloud in the lattice points of the structure
Forces set up between the unshared electrons
on a highly electronegative atom on one molecule and the weak positive charge from the ‘exposed proton of a hydrogen atom
Weak forces in all molecules that are associated with temporary fluctuations in electron density caused
by the rotation of electrons around atomic nuclei
Intermolecular forces set up between weak and electronegative charge on one polar molecule and electropositive charge on a second polar molecule
Very weak dipole-lie forces between non-polar molecules set up by weak dipoles induced by the proximity of other strongly polar molecules
Most molecules Polar organics
Non-polar organics
~~
150-250
15-1 70 100-200 27-83
<12
< 10
Trang 9116 Hydroblasting and Coating of Steel Structures
Table 5.2 Cohesion strength of substrates
Coatings
Gaughen (2000)
Relius Coatings, Oldenburg
Carbonline, St Louis
evaluated by different methods, including the following:
0
0 penknife disbondment;
0
0 falling ball impact
pull-off testing (IS0 4624; ASTM D4541);
cross-cut testing (ASTM 3359; DIN EN IS0 2409);
The pull-off test delivers quantitative information about the adhesion (usually given in N/mm2 or ma), while the picture of the rupture provides information about the weakest part of the system Typical failure types observed are either adhesion failure (substrate-coating) or cohesion failure (internal coating failure) Table 5.2 lists cohesive strength values of some metallic substrate materials More detailed designa- tion is mentioned in Table 5.3 Rigidly seen, a plain adhesion failure will not occur
This restriction is reinforced by XPS (X-ray photoelectron spectroscopy) measure- ments by de Vries et d (1983) who found traces of polymeric material on the substrate surface of a metal-polymer interfacial fracture which appeared to be
a purely adhesive failure from an optical examination
Desired adhesion depends on the certain case of application The US Navy, how- ever, has defined a general minimum pull-off strength of 3.4 MPa measured per
ASTM D4541 (Kuljan and Holmes, 1998)
5.2.2 Adhesion to Bare Steel Substrates
Several systematic studies have been performed to estimate the adherence of coating systems to steel panels prepared by different methods Long-term tests in salt water were performed by Allen (1997) and Morris (2000) These studies included hand wire brushing, needle gunning, hydroblasting and grit-blasting The results, listed in Tables 5.3 and 5.4, illustrate the complex relationships between preparation meth- ods and applied coating systems Cross-cut, measured after 36 months, was almost independent on the preparation method for many epoxy coatings; exceptions were coal tar epoxy and pure epoxy tank lining, where wire brushing and needle gunning showed worse results compared to hydroblasting and grit-blasting Penknife
disbondment and impact resistance, both measured after 24 months, showed worst
Trang 10Surfice Quality Aspects 1 I 7
Table 5.3
(Morris, 2000)
Results of comparative long-term adhesion tests after I t , 24 and 36 months
Method Cross-cut in mm Impact resistance’ Pull-off adhesion in MPaL
Solventless epoxy (2 X 12 5 p m DFT)
J
Glass flake epoxy (2 X 125 pm DFT)
Hydroblasting Dw3 FR 0 0 0 1 1 1 6.9/G 16.9/1 >17.2/1
Low temperature cure glass flake epoxy (2 X 125 pm DFT)
Modified epoxy (2 X 12 5 pm DFT)
~~
‘0 = no cracking: 1 = very slight cracking, no detachment: 2 = slight cracking, no detachment: 3 = lFailure mode: S =substrate: I = intercoat: G = glue
moderate cracking, no detachment
results for mechanical methods (especially for wire brushing) Impact resistance was more a function of the coating system than the Preparation method, thus grit- blasted substrate was, on the whole, only slightly superior to manual preparation under the conditions of impact testing Regarding the pull-off strength, measured with a commercial adhesion tester, blasting methods were superior to mechanical methods Some results are shown in Fig 5.1 There was a certain trend for blasting
Trang 1111 8 Hydroblasting and Coating of Steel Structures
Table 5.4 Results of comparative adhesion tests on ballast tank coatings (Allen, 1997)
Falling ball impact' Pull-off adhesion Penknife disbondment
Wire brushing
Needle gunning
Hydroblasting Dw2
Hydroblasting Dw2 FR
Hydroblasting Dw3
Hydroblasting Dw3 FR
Grit-blasting Sa 2 112
Wire brushing
Needle gunning
Hydroblasting Dw2
Hydroblasting Dw2 FR
Hydroblasting Dw3
Hydroblasting Dw3 FR
Grit-blasting Sa 2 112
Wire brushing
Needle gunning
Hydroblasting Dw2
Hydroblasting Dw2 FR
Hydroblasting Dw3
Hydroblasting Dw3 FR
Grit-blasting Sa 2 112
Wire brushing
Needle gunning
Hydroblasting Dw2
Hydroblasting Dw2 FR
Hydroblasting Dw3
Hydroblasting Dw3 FR
Grit-blasting Sa 2 112
2
1
0
4
0
0
1
4
3
0
2
0
2
1
2
2
2
0
0
0
0
2
1
1
4
0
0
0
Epoxy coating (solvent-less)
2.81s
2.81s 6.9lG 3.4lG 3.4lG
4.1lG
5.51G Coal tar epoxy
2.11s
2 4 6 5.211 6.911 6.911 6.911 6.611 Epoxy system
2.11s
2.81s 6.91G 5.51G 5.21G 6.9lG 5.51G Glass flake epoxy
2.81s 4.11s 6.91G 5.2lG 3.4lG 5.5IG 6.9lG
6
5
0
0
0
0
0
10
7
0
0
0
0
0
5
3
0
0
0
0
0
5
3
0
0
0
0
0
' 0 = no cracking no detachment: 1 = slight cracking, no detachment: 2 = slight cracking and detach-
*Failure mode: G = glue; I = intercoat; S = substrate
ment: 3 = moderate cracking, no detachment: 4 = moderate cracking, slight detachment
methods in that pull-off adhesion increased with time Under simulated ballast tank conditions, coatings applied to hydroblasted surfaces performed far better than coat- ings applied to mechanically prepared substrates, and equal to those on grit-blasted surfaces The influence of flash rust on coating performance was also investigated by Allen (1997) and Morris (2000) Selected results are shown in Fig 5.2 In more than half the cases, allowing the hydroblasted surfaces to flash rust reduced the