Metallization is the coating of a surface with metal by melting it with an electric arc. Plasma-boiled metal is sprayed with a stream of air of 5-7 bar on the surface to be metallized. Zinc, aluminum, copper, stainless steel and other molten metals, as well as alloys of these metals, are used for metallization. The surfaces of all materials that are insoluble in the sprayed metal can be metallized. It can be used for spraying sculptures with copper and restoration of work equipment parts spec. metallization of metals and glass surfaces, but the most widely used purpose of metallization is anti-corrosion coating. Metal parts of vessels above water, exterior of tanks.
Thermal spraying, or metallization, is regulated by an international standard – EN ISO 22063.
The standard interprets the coating of ferrous metals using metallization with zinc, aluminum, their alloy with 15% aluminum and 85% zinc; 95% aluminum and 5% magnesium.
The metal used for metallization adheres to the surface mechanically, so the standard provides for surface preparation up to Sa2.5 or Sa3 by abrasive blast cleaning. This is regulated by ISO 8501-1. The recommended surface roughness according to ISO 8503 – 2 is sharp and the height is from 40 μm.
METALLISATION AS AN ALTERNATIVE TO GALVANIZATION AND PAINT COATING:
The advantages and disadvantages of metallization compared to paint are set out in the following table:
|Significantly longer service life – from 20 to 50 years (maintaining the recommended thicknesses of ISO 22063).|
|Guarantees anodic protection||Sandblasting is required|
|Resistance to abrasion||Resistance to abrasion|
|Cheaper price per 1 m2 in terms of service life||Higher initial investment|
|Allows the structures to continue to be used immediately without interruption|
Unlike paint-coated surfaces, products with a heat-spray coating can be used almost immediately after work. Down-time and costs are further reduced if metal is sprayed on individual parts during production. The coating can be sprayed on any part of the product and it is possible to assemble the product immediately after metallization.
The cost of metallization is roughly equal to the cost of a good paint coating process, but considering the lifespan difference is 25 to 30 years, it is apparent that money is saved by choosing this method.
ADVANTAGE OF METALLISING COMPARED TO HOT GALVANIZING:
- It is resistant to mechanical shocks and abrasion. Hot-dip galvanizing is characterized by delamination.
- Large massive constructs can be metallized. The dimensions of hot-dip galvanized structures are limited by the dimensions of the galvanizing bath.
- Various metals can be used for metal protection during metallization. This guarantees anti-corrosion resistance in a variety of aggressive media. For example, aluminum is a weakly dilute acid deposited during metallization. (For example, during metalization the carried over aluminum holds onto weakly diluted acids.) (skliausteliuose pataisytas sakinys, nes mes su draugu niekaip nesugebejom suprasti originaliojo, galbut jis ir teisingas mes tiesiog nesuprantam terminu (: )
- Local locations can be repaired.
- No need to disassemble equipment.
- Can be coated up to 500 μm thick.
THE ADVANTAGE OF METALLISATION BEFORE PAINTING IS PROVEN BY REAL TESTS.
Many years of experience and testing of external surface effects have shown that metallization is more effective in protecting steel from corrosion than conventional surface coating with paint. Also, considering the service life, it is cheaper compared to painting. In addition, you will avoid a series of routine cosmetic repairs after 5 to 10 years and full repairs after 15 years. The American Welders Association (ASD) has been researching the corrosion process for 19 years. 1974 ASD completed a 19-year anti-corrosion protection study by spraying aluminum and zinc coatings on steel.
The results of the study are as follows:
0.08 – 0.15 mm thick aluminum coatings provide full anti-corrosion protection of the metal – substrate in 19 years of seawater Im2 and aggressive marine – industrial environment (C5-M);
The zinc coating must be 0.3 mm thick to guarantee complete surface protection in seawater for 19 years. In aggressive marine-industrial environments, a 0.23 mm coating of zinc without a top coat and a coating of 0.08-0.15 mm of zinc with a top coat guarantee 19 years of anti-corrosion protection.
In cases where physical damage such as scratches occurs after the use of the aluminum coating, corrosion does not progress at that location. This means that the anodic protection is active.
The types of coatings listed in Table no. 2, guarantees full protection of low-carbon steel for over twenty years.
|Table no. 2 Coating thickness required for thermal spraying. Source: American Society of Welders.|
|Type of exposure||Metalizuotas aliuminiu||Metalizuotas cinku|
|Impact of seawaterImpact of seawater||0,150 mm- uždažant
0.150 mm – by painting
|0.30 mm – uncovered|
|Marine atmospheric effects
|0.008 mm- when painting
0.15 mm – uncovered
|0.24 mm – uncovered
0.08 mm – by painting
|Industrial environment||0.08 mm- when painting
0.15 mm – uncovered
|0.24 mm – uncovered
0.08 mm – when painting
Metal spraying has already been been used 20 years ago, in coating a building in the North Sea. The building featured specialised equipment, such as signaling beacons, bridge decks between platforms and spare steps that could not be temporarily removed.
Areas that were difficult to access were metallized, such as lower platforms and metal water-filled areas, which are expensive to service. Now the whole platform, especially the part that pumps the gas, is protected by metallization.
One of the platforms that used electric spraying is in the settlement of Troll in Norway. Another Norwegian project, Soposo Heidrun, treated the surface of unprotected steel vessels and tanks in a humid environment (maximum operating temperature 1200C):
- surface preparation by abrasive blasting Sa3,
- AlMg5 spraying (200mkm +/- 50 microns) by electric arc,
- Layer of epoxy bonding coating (25 microns)
- A layer of polyurethane coating (50 microns).
In Caister, in the village of Murdoch, the structures were made using Sonos technology. Parts of the torches and lower deck platforms that were used for oil extraction from the seabed were sprayed with an aluminum coating. The minimum rate of aluminum adhesion is 7.0 mPa., with an average of 12.0 mPa after 20 years The required conditions for the use of the aforementioned coating are: relative humity (80% max) at a minimum steel and air temperature of 100 C. The coating was applied 20 years ago and is still performing exceptionally to this day.
WHAT METAL TO USE FOR METALLISATION
Sprayed metals such as cadmium, zinc and aluminum magnesium are chemically active. They become anodes for steel. When the coating is damaged, only that matterial is destroyed and the base (steel) stays untouched.
99.5% pure aluminum is used for metallization.
Resistant to marine and polluted environments. Suitable for anodic protection. According to ISO 22063, it is coated with 200 μm for immersion in salt and fresh water (Im 2 and Im3) and is not topcoated.
Resistant to high temperatures (can be used under fireproof coatings, outside pipes, steam tanks, etc.).
Forms an oxide layer, which reduces surface permeability;
Aluminum coatings are cheaper than zinc or any other matterial with the same thickness, but it requires a higher quality preparation of the surface to be coated (Sa3). Aluminum is not recommended for coatings smaller than 75 microns. Aluminum is also effective as an anti-corrosion coating under insulating materials that get wet when submerged. The use of heat-sprayed aluminum is recommended for factory equipment operating at high temperatures.
Sprayed aluminum is able to withstand the effects of many dilute acid solutions (pH 3.5 and above) and the service life of the coating depends on the chemical composition of the reagents and the type of surface. Heavily diluted solutions of nitric and sulfuric acids, as well as organic salts, have a negligible effect on aluminum-coated surfaces.
Zinc of 99.9% purity is used for metallization.
Suitable for anodic protection.
Excellent resistance to mechanical impact.
The service life is proportional to the thickness of the coating (in which case no paint is used).
A thin layer of 25 micron zinc guarantees excellent protection for small parts such as bolts and nuts. Consequently, the use of zinc coating is recommended in case it is necessary to keep the price to a minimum.
Zinc coating is not resistant to almost all acids, both organic and inorganic. Zinc coating is used when the pH range is from 6 to 12.
Zinc is more resistant in hard water. It is not resistant to any water if the water temperature exceeds 240C.
Zn / Al 15
It is the optimal combination of price and anti-corrosion resistance. Zinc 85% Aluminum 15%. According to ISO 22063, for immersion in fresh water (Im3), for the marine atmosphere (C5-M), for the industrial environment (C5-I; C4), a thickness of 150 μm without any top layer is sufficient.
AL / Mg5
An aluminum alloy with 5% magnesium is a great alternative to coating with good and expensive paints. The alloy is an effective coating for offshore platforms and parts that are on deck or submerged in water. According to ISO 22063, 250 μm of this coating will last for about 50 years.
By protecting the metallized surface with paint, we will have a service life of about 50 years.
When using metallization above 150 μm, the coating does not require a paint topcoat because aluminum and zinc naturally form a hydroxide film that closes the pores.
It is usually recommended to paint with at least one coat to fill in the open pores. Some projects use 3-4 micron coats of 3-4 paints. Typical painting examples are:
- Offshore (offshore modules under water): 1 layer 20 microns thick;
- Offshore (offshore modules above water): 1 coat of 25 microns thick and 1 coat of paint 40 microns;
- Steel containers for storing oil products and bridges: first layer 25 microns; 2 layers of epoxy coating 100 microns; top coating 50 μm acrylic polyurethane.
Used in the text:
Material from the UK Corrosion Institute in the UK.
- Corrosion resistant metal coatings.
- Corrosion protection in oil and gas installations and thermal spraying methods.
- Effective anti-corrosion protection of tanks using chemically active substances.
Standards to be followed when using the metallization process.
LST EN 22063 Metallic and other inorganic coatings – Zinc, aluminum and their alloys; The same – ISO 22063.
LST 5493 Protective coatings for iron and steel structures. Manufacture, transport, storage and installation.
DIN 8566 parts 1 and 2 on thermal spraying.
ISO 1463 Metallic and metal oxide coatings – Measurement of coating thickness – Microscopic 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 – Test of strength and adhesion of coatings.
ISO 8501 1-4 Preparation of steel substrates before application of paints and related products – Surface roughness characteristics – Steel substrates after abrasive cleaning.
SS 2626 Thermal Spray Equipment – Requirements and tests.