Steel structures Thermal ArcSpraying.

Company Vaitometa makes steel Thermal ArcSprayingFor this service please contact us by phone +37065279244

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Metal items blasting

UAB "Vaitometa" staff can carry out the new and old metal items blasting (abrasive cleaning). We paint all types of paint.

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2019 12 07

Steel structures Thermal ArcSpraying.

JSC ''Vaitometa’’ within own shop and/or in customer’s place are executing steel structures Thermal ArcSpraying.
For this service please contact us by phone +37065279244.
Electric-Thermal ArcSpraying  –  is a process of surface coating by using metal, which is melted by electric arc.  Metal transferred/turned into plasma is sprayed by 5-7 bar air flow on the surface to be metallised. For is using zinc, aluminium, copper, stainless steel and other metals melt, as well as their alloys. 
can be any materials surface which are insoluble or of decomposition from affect of sprayed metal. This can be spraying of sculptures surfaces by copper and equipment working parts restoration by special metal and glass surface metallisation, but the most widely applied metallization purpose is anti-corrosive covering. 

Thermal ArcSpraying

Economically metallisation of steel structures is better than simple painting, if life time of designed product is the more than 15 years.

Within Europe usage of metallization is increasing for anticorrosive protection of bridges and steel structures. Vessels above water areas, storage tanks from outside are metallised.

Thermal spray, or metallization, is governed by international standard EN ISO 22063.

 
This standard describes steel protection by by using zinc, aluminium, their alloys: 15% aluminium and 85% zinc; 95% aluminium and 5% magnesium. 
Metals used for adheres to the surface mechanically, therefore standard requires surface preparation to class SA2.5, SA3 by using abrasive blast cleaning. It is governed by the ISO 8501-1. Recommended surface roughness according to ISO 8503-2 is sharp, and the height of roughness peaks from 40 microns.

 

is an alternative to galvanizing and coating by paint: 

advantages and disadvantages compared with coating by paint are set in the following table: 

Advantages 

Disadvantages  

Much longer lifetime - from 20 to 50 years (after passing the ISO 22063 recommended thickness).

 

Guarantee the necessary anode protection

Sandblasting is necessary.

Resistance to abrasive effect

Operator skills

A cheaper price per square meter if calculated for life time period.

Higher primary investment.

Allows construction to be used without breaks as soon as possible.

 

 

 

Unlike after painting, metal coated structures can be used almost immediately after the execution of works. Time and money will be saved more effectively if certain parts of structures will be metal coated during prefabrication process. It is possible to spray metal coating on any item part and assemble products right after metallization is complete.

Price of is practically equal to price of painting by using paint of premium class, but if assess the difference of life time as 25-30 years longer we see that by using this method you save your money.

 

advantage over hot-dip galvanizing: 

1st – This is durability to shock and mechanical friction. Galvanizing typically peels off.

2nd – Large and massive constructions can be metallized. Galvanizing items are limited by galvanizing bath dimensions.
3rd – For metallization can be used various metals for steel protection. This ensures corrosion resistance in various aggressive environments. For example, surfaces metallised by aluminium are resistant to weakly diluted acids. 
4th – Localized areas can be repairs. 
5th – No need to disassemble equipment. 
6th – You can cover items with thickness up to 500 microns. 

advantages against painting are proved by real test. 

Long-term experience research and testing of the external forces influence to the surface has proved that the plating is more efficient steel protection against corrosion than the conventional coating by paint. Also, the assessment of life cycle, it is cheaper compared with painting. In addition, you will avoid intermediate cosmetic repairs after 5 and after 10 years of age and complete repair (major refurbishment) after 15 years. 
American Welding Society (ASD) has studied the corrosion process during 19 years. In 1974 ASD has completed the 19-year corrosion protection test, test by spraying of aluminium and zinc coatings on steel. 
The test results are as below: 
0.08 to 0.15 mm thickness metallised aluminium coating provides complete corrosion protection for steel base during 19-year in marine salt water (Im2 – ISO12944) and aggressive marine-industrial environment (C5-M – ISO12944);
Zinc coating should be 0.3 mm thick (not less), to ensure the full protection of the surface in marine water during 19th years. In aggressive marine - industrial environment zinc with 0.23 mm thickness without top coat and zinc with 0.08 to 0.15 mm thickness with top coat will ensures excellent protection for 19 years;
In case of aluminium coating usage if physical damage occurs, such like scratches, corrosion on the site is passive. This means that anodic protection is in action. 
Types of coatings, listed in Table. 2, over steel with low amount of carbon guarantees complete protection for over twenty years. 

Table. 2

Film thickness, required for thermal spraying. 
Source: American Welding Society. 

Exposure type

Metallised with aluminum 

Metallised with zinc

Exposure to sea water

     Im2

0,180 mm – if uncoated

0,150 mm – if coated

0,30 mm – if uncoated

Exposure to Marine atmosphere/environment

             C5-M

0,008 mm – if uncoated

0,15 mm – coated

 

0,24 mm – if uncoated

0,08 mm – coated

 

Industrial environment

           C5

0,08 mm – coated

0,15 mm – if uncoated

0,24 mm – if uncoated

0,08 mm – coated

 

Already 20 years ago, the first structure in the North Sea, which was covered with a metal spraying, were the specialized equipments, such as a signal flare beams, bridge span between the platforms and back-up steps, which could not been temporarily removed.
Metallized were difficult to access areas such as the underside of the platform and the steel splash zone, service of which is expensive. Now, the whole platform, especially the part that is pumping gas, is protected by metallization method.

 

Among the platforms, for which electric arc spraying was used - troll village in Norway. Other project in Norway, Heidrun Soposo, unprotected steel tanks and vessels, which are within moist environment (with a maximum operating temperature of 1200C), the surface was treated: 
• Surface preparation by abrasive blasting Sa3,;
• AlMg5 spraying (200mkm + / - 50 microns) by electric arc;
• coated by Binding epoxy coating layer (25 microns);
• coated by polyurethane coating layer (50 microns). 
Caister, Murdoch village, where construction has been made under Sonoso technology, torches and on the lower decks items of the oil production from the seabed platforms, have been sprayed aluminium coat. The minimum rate of adhesion of aluminium is 7.0 MPa, average - 12.0 MPa after 20 years.  Use conditions of those coverings - relative humidity (80% max) with a minimum steel and air temperature of 100 C. And it's done 20 years ago and now serves as well. 

WHAT METAL TO BE USED FOR METALLIZATION 

Those sprayed metals as cadmium, zinc, aluminium and magnesium are chemically active; they are anodes in pair with steel. If coat is damaged, it dissolves itself, and the base (steel), don’t. 

Aluminium: 
For is used 99.5% pure aluminium. 
Being resistant to marine and polluted environment it is suitable for anodic protection. According to the ISO 22063 aluminium is applied at 200 microns for immersion in salt or fresh water (Im2 and Im3) without top coat.
Is resistant to high temperatures (can be used in fire protection systems, exterior of piping, steam tanks...). 
Weight is light. 
It forms oxide layer, which reduces the transmission of the surface; 
Aluminium coating are less expensive compared to zinc or any other, with the same thickness, but if in use it needs a better quality surface preparation - Sa3. Aluminium is not recommended for use in less than 75 microns coating. Aluminium is effective as an anti-corrosive coating behind insulating materials, which condensates in presence of water. Heat sprayed aluminium is recommended to use for factory machinery, operating at high temperatures. 
Sprayed aluminium is able to withstand the most dilute acid solutions affect (with a pH of 3.5 and above), the life time of such a coating is determined depending on the chemical composition of the reagents and the type of surface. Strongly dilute nitric acid and sulphuric acid solutions, as well as organic salts have a negligible effect on aluminium coated surfaces. 

Zinc: 
For is used - 99.9% pure zinc. 
It is suitable for anodic protection. 
Has excellent resistance to mechanical impact. 
Life time is proportional to the thickness of the coating (in case if not over coated). 
25 microns a thin layer of zinc gives great protection of small parts, such as bolts and nuts. Thus, the zinc coating is recommended for use if necessary to minimize the cost. 
Zinc coating is not resistable to almost all, both organic and inorganic, acids exposure. Zinc coating is used when the pH range is from 6 to 12. 
Zinc resistance is higher in hard water. It is irresistible to any water, if the water temperature exceeds 240C. 

Zn/Al 15 
This is the optimal combination of price and anti-corrosion resistance. Zinc 85% Aluminium 15%. According to ISO 22063 for immersion in the fresh water (Im3), the marine atmosphere (C5-M), industrial environment (I-C5, C4) - it is enough 150 microns thickness of this alloy which is uncovered by any top coat 

AL/Mg5 

The use of aluminium alloy with 5% magnesium, it is a great alternative to expensive and good coating of paint. That alloy is an effective coating for marine platforms and components, which are on ships deck or are immersed in the water. According to ISO 22063 250 micron of that coating will serve about 50 years. 

If surface will be secured by paint, we will have about 50 years of time in service. 
Using metallization over 150 microns, overcoating may be not used, because aluminium and zinc are naturally forming hydroxides film, which closes the pores. 
Usually is recommended to paint at least one layer coat to fill the open pores. Some projects are using 3-4 layers of paint with 250 microns total dry film thickness. Typical examples of paintings are: 
• coastal waters (off-shore models) (in water): 1 coat with 20 micron thickness; 
• coastal waters (off-shore modules) (parts above water): 1 coat with 25 microns thickness and 1 layer of 40 microns covering paint; 
• Steel containers for storage of petroleum products and bridges: The first layer is 25 microns; the second layer is epoxy coating with 100 microns; the top coat is acrylic polyurethane with 50 microns. 

 

Within the text has been used these sources: 
British Institute of Corrosion materials. 
Articles: 
1. Corrosion-resistant metal coatings. . 
2. Protection against corrosion of oil and gas installations companies and techniques of thermal spraying. 
3. Effective anti-corrosion protection of tanks when used chemically active materials. 

The standards, which should be used during metallization process. 

LST EN 22063 Metallic and other inorganic coatings - Zinc, aluminum and their alloys; Same - ISO 22063.
LST 5493 protective anticorrosive coatings for iron and steel structures. Manufacturing, transporting, storage and installation. 
DIN 8566 Parts 1 and 2 the thermal spraying. 
ISO 1463 Metallic and metal oxide coatings - Measurement of coating thickness – microscopical method.
ISO 2064 Metallic and other inorganic coatings - Definitions and symbols.
ISO 2176 non-magnetic coatings on magnetic surfaces - measurement of coating thickness - Magnetic method. 
ISO 4624 Paints and varnishes – pull off test for adhesion. 
ISO 8501 1-3 Preparation of steel substrates before application of paints and related products - Visual assessment of surface cleanliness. 
HS 2626 Thermal Spray Equipment - Requirements and tests.