Learn how this corrosion protection system is being used in industrial atmospheric environments, with minimal environmental impact
by Abel de la Cruz*
As is known, one of the most widely used metal coatings in the atmospheric corrosion protection of steel is galvanized, because it has the property of forming a compact film consisting of its corrosion products. On the other hand, since zinc is more active than iron, it gives rise to the formation of a galvanic pair with steel, decreasing its rate of deterioration.
There are two basic methods to obtain galvanized steel and from there a whole range of products are derived that make it a product of multiple possibilities, that is, more versatile:
Hot dip: the process basically consists of immersing the steel to be coated in a vat where the molten zinc is located. Different types of zinc alloys are used with other metals.
Electrodeposition or electrolytic galvanizing: Unlike the hot immersion system, here the process to apply zinc uses electric current in an electrochemical system.
Due to the method or procedure of obtaining the coating, it turns out that its use has been limited for new construction works where structures, parts, parts, machines or equipment have had to receive the treatment previously in the workshop or in the production plant.
However, the development of these coatings in the form of zinc tapes designed to coat metal structures, preserving them and protecting them from atmospheric corrosion, constitutes a new alternative for the maintenance of corrosion protection on site and in the long term, projecting durability of more than 15 years in industrial atmospheric environments.
Secondly but not least, it turns out that the use of the zinc tape system carries a minimal environmental impact, as it does not send volatile particles into the environment as is the case with many of the paint systems.
The system consists of a high purity zinc tape (99.99% zinc), an electro-conductive adhesive and a silicone paper coating, as protection against contamination until the time of installation.
The system provides double protection to new or existing structures, mainly made of steel, but also of iron, aluminium and noble alloys. First of all, and like a paint, the tape acts as a barrier to the passage of contaminants; it also provides galvanic (cathodic) protection to the coated base metal, which remains corrosion-free for a longer time. The basis of this cathodic protection is explained because in the presence of moisture, zinc, being more electronegative, acts as an anode or sacrificial element before the steel or cathode, which is protected.
It is important to highlight the function of the adhesive layer of the system, formed by a binding matrix with zinc powder, which acts as an electrical coupling necessary for metal continuity, between the surface of the steel to be protected and the zinc tape.
The barrier protection provided by paints, especially in metal structures exposed in contact with environments of high corrosive aggressiveness (Marine/-Industrial), has the disadvantage of allowing the access of polluting agents and a certain degree of humidity that translates into the beginning of the oxidation process, once there is rupture or another form of damage to the paint.
In the case of zinc tape, the above does not happen, since if the zinc layer is damaged, scratched or discontinuous, the adjacent zinc will form a protective compound, sealing the pores or cracks produced and, finally, protecting the surface against corrosion.
The electro-conductive adhesive of 0.025 mm (1 thousand, 25 microns) and the tape with a nominal thickness of 0.080 mm (3 mils, 80 microns), together with the zinc content of 99.99%, give the system the cathodic protection characteristics required to protect the steel, offering a corrosion resistance equal to or better than that of hot dip galvanizing.
Tests to assess corrosion protection
Accelerated corrosion tests in the laboratory, immersion tests and salt spray (ASTM B117) for more than 1000 hours using 5% NaCl solutions, reveal that zinc tape protects the substrate metal (carbon steel in this case), from external corrosive attack.
The tape primarily acts as a physical barrier between the metal and the environment and is sacrificed to protect the base metal, which is evidenced by the white zinc oxide found in the exposed samples. In addition, these laboratory tests found that zinc tape shows better performance than hot-dip galvanized, which is indicated by the presence of dark red oxide in galvanized steel samples, while zinc tape-coated panels show a uniform coating product of white zinc oxide, after being exposed to the salt spray test.
This is attributed to the uniformity of thickness in the zinc tape, which is not found in hot dip galvanizing, as well as the presence of inclusions, difficult to control in the latter system.
The zinc tape system has proven to be a good option for protection against atmospheric corrosion in aggressive environments such as industrial and marine environments. Research developed in Kuwait (Kuwait Institute for Scientific Research) in 2005, to determine the durability and corrosion resistance of zinc tape coating, reveal positive results for the system.
Samples of the tape on rack racks are exposed to industrial environment (Shuaiba Area) and marine environment (Failaka Island), observing to date good behavior of protection against corrosion. These samples continue to be investigated and monitored for 3, 6, and 10 years of exposure.
Installation methods
For flat areas or of a certain extension, such as large volume storage tanks, a magnetic roller is used to exert sufficient pressure on the substrate, ensuring good adhesion and preventing the formation of wrinkles / bubbles.
Pipe protection requires to be executed quickly and efficiently; for this, the spiral winding winding machine (Metawraper) is used, selected according to the diameter, which ensures the required tension in the tape, as well as the pressure of the rollers to obtain an application free of wrinkles and bubbles.
When the shape of the element to be protected does not allow the use of the winding machine, as in the case of a tank, strips of tape of equal length to the circumference of the tank are cut. The tape is then secured to the surface using the magnetic roller.
Uses
The main applications of the system are in the following industries:
1. Construction industry. For the protection of metal structures such as bridges, lighting poles, energy transmission towers, communications, and structures of chemical machinery, water treatment, recreation, agricultural and mining.
2. Automotive industry. Edge protection on window frames. At the interface of support structures (chassis), iron in trains receiving noble metal bodywork, doors, sites of stagnation or water infiltration in cars, trucks and trains, as well as hinges in cargo containers; cases in which the zinc tape is cut to obtain the required profile.
3. Oil industry such as pumping stations and refineries. The high standards of this industry, demand cathodic protection by printed current to buried ducts; however, this corrosion protection ceases to have an effect when the pipes leave the ground (electrolyte) and come to the surface (atmosphere). In these environments of atmospheric corrosion, the zinc tape system protects the pipes by transporting different fluids, as well as storage tanks for water, gas and refined products, loading arms, pipe racks, valve bodies, flanges and metal elements typical of this industry.
4. Port and naval industry. Like the oil industry, the demands in these industries point to the best maintenance of their structures to avoid unsafe conditions that put personnel, facilities and the environment at risk. This is how zinc tape is used in the protection of metal piles in the atmospheric zone, structural profiles, pipes subjected to condensation phenomena, or in the maintenance of structures previously protected by hot dip galvanizing and damaged by the action of welds, cuts, burns, shearing, or transport, cases in which the application of zinc tape is easily applicable in the field by the user.
Reference Bibliography:
Technical Information and Application Methodology of Vector Corrosion Technologies- Canada.
www.vector-corrosión.com
* General Manager- Principal Consultant. AmericanConsult Peru. [email protected]. http://www.americanconsultperu.com
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