The degree of oxidation of a steel surface refers to the conditions of a surface before being cleaned. The degree of surface preparation corresponds to the final cleaning of the surface prior to the application of protective coatings.
Among the existing surface preparation standards, the most common in Brazil at present is ISO 8501-1, but the Swedish Standards Institution (SIS 05 5900) of 1988 can still be specified, even though it has been canceled. SIS has joined ISO and they unified their visual standards and corresponding names.
The box below shows equivalent standards to the above mentioned.
| TREATMENT |
ISO
8501-1 / SIS 055900 (1) |
SSPC
(3) |
BS 4232
(4) |
NACE
(5) |
JSRA
SPSS (6) |
|---|---|---|---|---|---|
Power Tool Cleaning |
St2 |
SP2 |
- |
- |
- |
Power Tool Cleaning |
St3 |
SP3 |
- |
- |
Pt3 |
Light Brush-Off Blast Cleaning |
Sa1 |
SP7 |
- |
N° 4 |
- |
Commercial Blast Cleaning |
Sa2 |
SP6 |
3ª quality (80% min) |
N° 3 |
Sh1, Sd1 |
Near-White Metal Blast Cleaning |
Sa2½ |
SP10 |
2ª quality (95% min) |
N° 2 |
Sh2, Sd2 |
White Metal Blast Cleaning |
Sa3 |
SP5 |
1ª quality (100%
min) |
N° 1 |
Sh3, Sd3 |
Solvent Cleaning |
- |
SP1 |
- |
- |
- |
Flame Cleaning |
F1 |
SP4 |
- |
- |
- |
Pickling |
- |
SP8 |
- |
- |
- |
Weathering followed by Blast Cleaning |
- |
SP9 |
- |
- |
- |
Products formed as a result of white corrosion of zinc should be removed with water under high pressure or hand sanding. Brush-off blast cleaning may be used, followed by washing with water, to ensure the removal of soluble zinc salts. After cleaning and drying the substrate, apply a high-adhesion primer (REVRAN PAA 540 or REKOVIN PWP 588), suitable for non-ferrous surfaces.
- St 2 Standard: Hand Cleaning
Completely scraped steel surface treated by hand or power tools with a steel brush, etc. The treatment should remove loose mill scale, rust and any other material.
The surface should be immediately cleaned with a vacuum cleaner, dry compressed air or a hairbrush. The appearance should correspond to a soft metallic shine, according to the photo standards of St 2 designation.
This treatment does not apply to surfaces with ‘A’ degree corrosion. Achieved standards are: B St 2, C St 2 and D St 2 of ISO 8501-1 Standards.
- St 3 Standard: Hand Cleaning
Completely scraped steel surface treated by hand or power tools with a steel brush, grinder, etc, in a more thorough and harsh way than in the previous case. After dust removal, the surface should have a light metallic shine. The appearance should correspond to St 3 designation photos.
This treatment does not apply to surfaces with ‘A’ degree corrosion. Cleaning standards are: B St 3, C St 3 and D St 3 of ISO 8501-1 Standards.
This consists of a better steel surface preparation with the use of steel pellets or other abrasives.
- Sa 1 Standard: Light abrasive blast cleaning (BRUSH-OFF)
Loose mill scales, rust and foreign matter should be removed. The surface should be immediately cleaned with a vacuum cleaner, dry, clean compressed air or a clean brush. The final appearance should correspond to photo and visual standards, as per Sa 1. This treatment does not apply to surfaces showing ‘A’ degree weathering. For all others, treatment standards are: B Sa 1, C Sa 1 and D Sa 1 of ISO 8501-1 Standards.
- Sa 2 Standard: Commercial abrasive blast cleaning
Practically all mill scale, oxides and other impurities are eliminated by this treatment. If the surface has cavities (craters), at least 66.7 % of the 6.45 sq. cm area should be free of visible residues. The surface should be immediately cleaned with a vacuum cleaner, dry and clean compressed air or a clean brush. The final appearance should correspond to photo and visual standards, as per Sa 2. This treatment does not apply to surfaces showing ‘A’ degree weathering. For all others, treatment standards are: B Sa 2, C Sa 2 and D Sa 2 of ISO 8501-1 Standards.
- Sa 2½ Standard: Near-white metal abrasive blast treatment
Mill scales, rust and foreign matter should be removed in such a perfect way that its traces will only appear as light stains or streaks. The surface should be immediately cleaned with a vacuum cleaner, dry and clean compressed air or a clean brush. The surface should have an appearance that corresponds to photo and visual standards, as per Sa 2½, with its respective degrees of weathering: A Sa 2½, B Sa 2½, C Sa 2½ and D Sa 2½ of ISO 8501-1 Standards.
- Sa 3 Standard: White metal abrasive blast cleaning
Mill scales, rust and foreign matter should be removed completely, which will give a light gray appearance, free from stains or streaks. The surface should be immediately cleaned with a vacuum cleaner, dry and clean compressed air or a clean brush. The final appearance should correspond to photo and visual standards, as per Sa 3, with its respective degrees of weathering: A Sa 3, B Sa 3, C Sa 3 and D Sa 3 of ISO 8501-1 Standards.
Before abrasive blasting, the steel should be freed of fats, greases or oils, by degreasing with solvent-soaked cloths or rags, solvent vapors or other methods that will eliminate these contaminants.
Weld lines and sharp edges should be ground to prevent thin paint thickness on sharp edges, lowering their protection. It is practically impossible to uniformly cover welding splashes, which should be removed to prevent premature coating failure.
The roughness profile obtained from abrasive blasting will depend on the type and size of abrasive used, air pressure and applied technique. In general, the ideal profile for a coating system is one that encompasses about 30% of the final system thickness.
Abrasive |
Maximum Size of Particle that passes through mesh |
Maximum Profile Height (µm) |
|
|---|---|---|---|
Nominal Opening, mm |
Note
1 |
||
Steel pellets (angular particles) |
|||
Nº 40 – G 80 |
0,42 |
40 |
60 |
Nº 25 – G 50 |
0,7 |
25 |
85 |
Nº 18 – G 40 |
1,0 |
18 |
90 |
Nº 16 – G 25 |
1,2 |
16 |
100 |
Nº 12 – G 16 |
1,7 |
12 |
200 |
Steel pellets (spherical) |
|||
Nº S – 110 |
0,6 |
30 |
50 |
Nº S – 230 |
1,0 |
18 |
80 |
Nº S – 280 |
1,2 |
16 |
90 |
Nº S – 330 |
1,4 |
14 |
95 |
Synthesized Aluminum Oxide |
|||
- |
0,4 |
40 |
80 |
The flame cleaning includes, at the end of the process, mechanical cleaning using wire brushes, to eliminate the products resulting from the flame cleaning.When closely examined, the surface shall present no mill scales, rust, paint, and any other material.
The treatment visual standards are: AFl, BFl, CFl e DFl of ISO 8501-1 Standards.
One of the standards used for this treatment is SSPC-VIS 5 / NACE VIS 9, that adopts visual standards known as WAB - wet abrasive blast cleaning.
Wet abrasive blasting is nothing more than blasting with dry abrasive mixed with water.
This method originated from the need to eliminate risks generated by conventional dry blasting, which can cause serious health problems to people (i.e. a professional disease known as silicosis).
NOTE: silicosis is an occupational disease caused by the inhalation of silica dust, which results from sandblasting. In this process, the sand breaks into particles when it hits metal surfaces thus releasing silica dust. As sandblasting is prohibited in Brazil, Sinterball synthesized aluminum oxide may be used as an alternative.
Although it achieving less cleaning than dry blasting, wet abrasive blasting is no doubt more efficient than mechanical treatment, which does not remove existing contaminants. With wet abrasive blasting, the profile is similar to dry blasting and we know what the contaminants present are: water, corrosion inhibitor and flash rusting.
Flash rusting is the light corrosion formed when the surface, which is blasted with no corrosion inhibitor, starts to dry. It may also happen after fresh water washing, with no corrosion inhibitor, of a surface blasted with wet abrasive, with a corrosion inhibitor.
- NO FLASH RUST
A surface which, when viewed without magnification, exhibits no visible flash rust.
- LIGHT FLASH RUST (L)
A surface which, when viewed without magnification, exhibits small quantities of a yellow / brown rust layer through which the steel substrate may be observed. The rust of discoloration may be evenly distributed or present in patches, but it is thightly adherent and not easily removed by light wiping with a cloth.
- MODERATE FLASH RUST (M)
A surface which, when viewed without magnification, exhibits a layer of yellow / brown rust that obscures the original steel surface. The rust layer may be evenly distributed or present in patches, but it is reasonably well adherent and lightly marks a cloth that is lightly wiped over the surface.
- HEAVY FLASH RUST (H)
A surface which, when viewed without magnification, exhibits a layer of heavy red / brown rust tha hides the initial surface condition completely. The rust may be evenly distributed or present in patches, but the rust is loosely adherent, easily comes off, and leaves significant marks on a cloth that is lightly wiped over the surface.
This standard describes two conditions of degree of oxidation:
- Condition C: - Steel surface whose mill scale has been removed by corrosion, or can be removed by scraping, showing small cavities. (Corrosion degree C);
- Condition D: Steel surface whose mill scale has been removed by corrosion, showing severe cavity corrosion. (Corrosion degree D).
WAB 10 (Near-white metal abrasive blast treatment) and WAB 6 (Commercial abrasive blast treatment) are used to classify the degree of cleaning or the degree of surface treatment.
Select the visual standard that mostly represents the initial condition of the material.
Select the reference photograph that mostly represents the specified surface preparation. Example: Rust degree D specified, Near-white metal abrasive blast treatment, use reference photograph D WAB 6.
Immediately after waterjetting, but before the surface has flash rusted, the metal must present the degree of cleaning. Example: WAB 6 or WAB 10.
Prior to painting, compare the flash-rusted surface with the flash rust photographs.
Example: If initial condition is D, use D WAB-6 L, D WAB-6 M, D WAB-6 H to evaluate the degree of flash rust with the flash rust photographs.
NOTE: in case of wet abrasive blast, the final inspection must be performed when the surface is totally dry. When comparing it with the reference photograph, the maximum the degree of cleaning may present is a very light, well adhered superficial rust. If primer is not residual humidity or wet surface tolerant, or if the surface presents moderate or heavy flash rust, it might be necessary to ask the paint manufacturer for technical help. In case the product does not meet the requirements, an additional treatment must be performed using steel brush, or hydroblast with minimum pressure of 21 Mpa (3.000 psi).
The use of wet abrasive blast requires conceptual change concerning the final appearance of the surface to be painted. For decades blasted surfaces should not present corrosion before being coated. Nowadays, when using wet abrasive blast without corrosion inhibitor, the surface presents flash rust, which is perfectly acceptable, provided proper products are employed.
Corrosion inhibitor is neither recommended for surfaces submitted to continuous immersion nor for rich zinc paints.
The application of REVRAN WBC 524, REZINC WBC 522 or REVRAN ECO NVC 997, REVRAN NVC WST is recommended for surfaces that present residual humidity after wet abrasive blast.
Due to the new approach of wet abrasive blast, it is recommendable our customers contact our technical consultants for further information.
Hydroblasting is a good technique of surface cleaning, which consists of using water at ultra high pressure to obtain an adequate painting surface. As no abrasive agents are used during the hydroblasting process, there is no generation of finely divided particles, being silica the most critical of all.
This cleaning system is ideal for areas submitted to certain restrictions, such as:
- Pollution caused by the suspension of abrasive particles;
- Equipment and processes contaminated by sand;
- Operational difficulties in chemicals plants as well as on offshore platforms, where processes shall not be interrupted, etc.
Hydroblasting is a surface treatment using high pressure water, and it is also known as water jetting, hydrojetting and water blasting.
According to NACE / SSPC – SP – 12 definition, hydroblasting is a process that consists of water been forced through a nozzle at pressures superior to 100.000 psi, in order to clean the surface to be treated.
Normally the high pressure hydroblasting (HPH) is divided into two operating categories, with pressures varying from 10.000 to 25.000 psi (680 to 1700 bar), and the ultra high pressure hydroblasting (UHPH), with pressures above 25.000 psi (above 1.700 bar).
It is difficult to determine the standards of hydroblasting surface cleaning. Some surfaces, for example, are prepared with dry abrasive blast, according to the treatment standards of the ISO 8501-1 Standard.
The most beneficial characteristic of hydroblasting is the substantial reduction of salts, mainly chlorine, and the removal of other water soluble materials. Due to osmosis, bubbles may appear on the film when salts are not removed from the surface before applying the painting system.
The SSPC-VIS 4 / NACE VIS 7 Standards contains visual standards of hydroblasting surface preparation for surfaces with aged coatings, as well as with weathering degrees C and D, according to ISO 8501-1 Standard.
The reference photographs illustrate five of the 7 initial conditions before surface preparation.
- Condition A (not illustrated): steel surface completely covered with adherent mill scale; little or no rust visible.
- Condition B (not illustrated): steel surface covered with both mill scale and rust.
- Condition C: - steel surface the mill scale of which has been removed by corrosion or that can be removed by scratching; little pitting visible (Rust Grade C);
- Condition D: steel surface the mill scale of which has been removed by corrosion; pitting visible (Rust Grade D);
- Condition E: previously painted steel surface; light-colored paint applied over blast-cleaned surface; paint mostly intact.
- Condition F: previously painted steel surface; zinc-rich paint applied over blast-cleaned steel, coatings slightly aged and paint mostly intact.
- Condition G: painting system applied over mill scale bearing steel; system throughly weathered, throughly blistered, or throughly stained.
- Condition H: degraded painting system applied over steel. System throughly weathered, throughly blistered, or throughly stained.
Conditions A and B are not illustrated because their initial and final condition are very similar.
Note: Hydroblasting does not remove scales if they are thightly adhered on conditions A and B. As a consequence, it does not generate anchor profile.
Steel to be cleaned may contain more than one of the initial conditions.
Select the photograph of initial condition (C or D) that most closely represents the appearance of the unpainted steel to be cleaned.
Select the photograph of initial condition (E, F, G or H) that most closely represents the appearance of the painted steel to be cleaned.
Table 1 refers to initial conditions and to the degree of cleaning corresponding to each condition.
Immediately after waterjetting, but before the surface has flash rusted, compare the prepared surface with the photograph and it should conform to WJ1, WJ2, WJ3 and WJ4.
Before painting the surface compare the degree of flash rust with the photographs. Table 2 lists the degrees of flash rust for initial conditions C and D.
- WJ-1 – The surface must be free of rust, paint, mill scales or any other contaminants previously visible, and present a flat matt metal finish.
- WJ-2 – The surface must be clean, present a matt finish, with at least 95% of the area free of all previously existing visible residues and the remaining 5% containing only rust stains, paint and contaminants at random.
- WJ-3 – The surface must be clean, present a matt finish, with at least 2/3 of the area free of all previously existing residues (except mill scales) and the 1/3 remaining containing only rust stains, paint and contaminants at random.
- WJ-4 – The surface must be flat, free of rust, paint and mill scales not adhered.
The steel surface presents variations of texture, shadows, color, shade, pits and scales, that have to be considered when comparing the reference photographs. Appearance variations are acceptable provided they do not affect the surface cleaning, including those caused by type of steel, surface original condition, steel thickness, weld, manufacturing process, thermal treatments, thermically affected zones, and differences resulting from prior abrasive superficial treatment.
The various degrees of cleaning, with no flash rust, are described in SSPC-SP 12 / NACE Nº 5.
Table 1
List of Reference Photographs (no Flash Rust) Showing Various Initial Conditions with 4 Degrees of Cleaning
| Initial Condition |
Condition C 100% Rust |
Condition D 100% Rust With Pits |
Condition E |
Condition F |
Condition G |
Condition H |
|
|---|---|---|---|---|---|---|---|
Degree of Cleaning |
WJ-1 |
C WJ – 1 |
D WJ – 1 |
E WJ – 1 |
F WJ – 1 |
G WJ – 1 |
H WJ – 1 |
WJ-2 |
C WJ –2 |
D WJ –2 |
E WJ –2 |
F WJ –2 |
G WJ –2 |
H WJ –2 |
|
WJ-3 |
C WJ – 3 |
D WJ – 3 |
E WJ – 3* |
F WJ – 3 |
G WJ – 3 |
H WJ –3 |
|
WJ-4 |
C WJ – 4 |
D WJ – 4 |
E WJ – 4 |
F WJ – 4 |
G WJ – 4 |
H WJ –4 |
|
* E WJ-3 ALT there is a photograph that illustrated this condition.
Table 2
List of Reference Photographs Showing Levels of Flash Rust*
Surface Initial Condition |
||||
|---|---|---|---|---|
| Condition C 100% Rust |
Condition D 100% Rust With Pits |
|||
| Degree of Cleaning |
WJ-2 |
WJ-3 |
WJ-2 |
WJ-3 |
No Flash Rust |
C WJ-2 |
C WJ-3 |
D WJ-2 |
D WJ-3 |
Light Flash Rust |
C WJ-2 L |
C WJ-3 L |
D WJ-2 L |
D WJ-3 L |
Moderate Flash Rust |
C WJ-2 M |
C WJ-3M |
D WJ-2 M |
D WJ-3M |
Heavy Flash Rust |
C WJ-2 H |
C WJ-3 H |
D WJ-2 H |
D WJ-3 H |
* Photograph references illustrate two initial conditions (degree of corrosion).
It is important to mention that this Guide describes the several conditions of a surface before being hydroblasted, that can vary from Rust Degree C to painted surfaces, and also of those surfaces coated with different types of paints.
By controlling the pressure, it is possible to selectively remove the paint film of coated surfaces, thus removing only the damaged coats.
The hydroblasting surface treatment does not produce roughness profile on new steel that presents mill scales.
NOTE: in case of wet abrasive blast, the final inspection must be performed when the surface is totally dry. When comparing it with the reference photograph, the maximum the degree of cleaning may present is a very light, well adhered superficial rust. If primer is not residual humidity or wet surface tolerant, or if the surface presents moderate or heavy flash rust, it might be necessary to ask the paint manufacturer for technical help. In case the product does not meet the requirements, an additional treatment must be performed using steel brush or hydroblast with minimum pressure of 21 Mpa (3.000 psi).