Process summary of supersonic spraying!

       Supersonic spraying thermal spraying durable anti-corrosion coating process

I. Overview

      Due to the long-term exposure of steel structures to various atmospheres (including oceans, industrial cities, fields, mines) and different water quality media, they are all subject to different degrees of corrosion. It is understood that about one tenth of the steel produced worldwide will rust every year, and about 30% of steel equipment will fail due to corrosion. In industrially developed countries, the direct economic loss caused by metal corrosion accounts for 2-4% of the total output value of the national economy.

Supersonic arc spray thermal spray coating of zinc, aluminum and their alloys, combined with sealing treatment, can produce long-term protective composite coatings. The effective protection period is generally up to 20-30 years, while the longest period of validity for foreign records is up to 60 years. With the development and improvement of electric arc spraying thermal spraying technology in my country, zinc, aluminum and other alloy coatings will gradually replace the traditional spraying process. In some areas, supersonic arc spray thermal spray zinc and aluminum coatings will also replace most hot-dip galvanizing processes. Here, the performance of the long-term anti-corrosion process is briefly introduced by taking the ultrasonic thermal spraying zinc-aluminum coating as an example.

 

Second, the performance of zinc-aluminum long-term protective composite coatings

1. Protection mechanism of zinc and aluminum coatings

      Studies have shown that the average corrosion rate of carbon steel is 5-20 times higher than that of zinc and 40-100 times higher than that of aluminum due to electrochemical corrosion of steel in atmospheric and aqueous media without protective measures. When common coatings are used for protection, localized coating loss and peeling are often easily caused due to the low adhesion strength of the coating to the steel substrate, low resistance to erosion and particle abrasion, and aging and deterioration during use, which can lead to The steel substrate rusts, which causes the paint layer to blister and peel off in large areas, causing the protection to fail in a short period of time.

The supersonic arc spraying thermal spraying zinc-aluminum composite coating has a double protective effect on the steel substrate.

On the one hand, the coating after sealing treatment can play a physical covering role. It isolates steel from corrosive media such as water and air, and plays a protective role; on the other hand, when the coating has pores, cracks or damage, the electrochemical performance of zinc and aluminum is relatively active, and the electrode potential is lower than that of steel. In forming a corrosion cell, zinc or aluminum is the negative electrode. Iron is the positive electrode. Zinc and aluminum lose electrons and become ions into the electrolyte and corrode. Free electrons flow to the steel, polarizing and protecting it.

Aluminum is a very reactive metal that easily combines with oxygen to form a dense and hard oxide protective layer, which effectively prevents further oxidation of the aluminum coating. In addition, during the spraying process, the aluminum becomes a negatively charged activated state, which allows for better cathodic protection of the steel substrate.

After spraying, use a low viscosity paint, seal the paint and add one or two top coats to become a metal matrix composite paint, which will further significantly improve the paint's corrosion resistance and extend the protection period.

Since zinc or aluminum is sprayed on the rough steel surface as a molten liquid in the supersonic arc spraying (thermal spraying) process, it melts with greater thermal energy, which is more susceptible to oxidation than organic coatings. In addition, during the spraying process, the aluminum becomes a negatively charged activated state, which allows for better cathodic protection of the steel substrate.

      After spraying, apply a low viscosity paint, seal the paint and add one or two top coats to form a metal matrix composite paint, which will further significantly improve the paint's protective ability and prolong the protection period.

2. Characteristics of zinc-aluminum coating by supersonic arc spraying

Structural steel components are protected by sprayed zinc and aluminum coatings and seal coats to form a long-term protective composite coating. Compared with general organic coatings, it has the following outstanding features:

(1) Long protection period: spraying zinc, spraying aluminum composite coating, the protection period is generally up to 20-30 years, the longest period of validity in the foreign atmosphere is a record 60 years, and the organic protection period is generally 3 years. -8 years.

(2) The bonding strength is higher than that of organic coatings, reaching 2-3kgf/mm2.

(3) High temperature resistance: The sprayed aluminum coating can resist oxidation at a high temperature of 900°C after diffusion treatment; the zinc coating can also withstand a temperature of 450°C.

(4) There is no problem of aging and deterioration.

(5) Good wear resistance, erosion resistance and scratch resistance.

 

3. Comparison of sprayed zinc and sprayed aluminum coatings

Zinc spray coating is the earliest cathodic protection coating applied abroad. Before the 1960s, most of the long-term protection used zinc spray coating. However, since (1) the zinc layer is a sacrificial anode material, the anti-corrosion rate of the zinc-sprayed coating is higher than that of the aluminum-sprayed coating, and its corrosion resistance period is proportional to the thickness. There is a certain thickness; (2) the density of zinc (7.09/cm3). It is 2.6 times higher than aluminum (2.7cm3), so the consumption of zinc is 2.6 times that of aluminum by spraying the same thickness of coating. Foreign data reported that the cost of zinc is twice as expensive as that of aluminum when spraying with an equivalent coating thickness; (3) the ZnO dust formed by spraying zinc is harmful to the human respiratory tract; (4) when spraying zinc, CO The content of ZnCO3 is insufficient, and a stable ZnCO3 protective film cannot be formed, and the easily loose Zn(OH)2 produces a lot of "white rust" on the zinc coating, and the corrosion rate increases; (5) Due to the SO2 in the atmosphere of industrial areas and cities The higher content increases the acidity of rainwater, which can also destroy the stability of the ZnCO3 film and accelerate the corrosion rate of the zinc coating.

With the development of new arc wire spraying technology, aluminum sprayed long-lasting protective coatings are widely used in large steel components in the world. In addition to continuing to be used in rural atmosphere, talking water and alkaline atmosphere, sprayed zinc coating is gradually being replaced by sprayed aluminum coating.

The corrosion rate of sprayed zinc and sprayed aluminum coatings is closely related to the PH value of the environmental medium used. Generally speaking, zinc spray coating is better for weak alkaline conditions, and aluminum spray coating is best for neutral or acidic conditions.

3. Typical domestic application examples of supersonic arc spraying zinc and aluminum composite layers:

(1) In 1920, the Saint-Guigny lock near Paris, France, was protected by spraying zinc, and the inspection was carried out in 1952, and the spray coating was intact.

(2) In 1936, the United States sprayed a zinc coating with a thickness of 250 um on the steel structure of the KAW River Bridge, and in 1975, the coating was inspected in good condition.

(3) In 1939, the suspension chain and some steel structure parts of the McNay Channel Road Bridge in North Wales, England were sprayed with zinc for protection, and the steel structure has been kept in excellent condition.

(4) The purification device of a natural vaporization plant in Glanwich, the United States, was sprayed with a 130um-thick aluminum coating on the steel structure, which remained in good condition after 30 years.

(5) For the 264 transmission towers of Huainan Power Plant in my country, the upper half of them were protected by spraying zinc in 1952, and the coating is still intact.

(6) The 63-hole steel sluice gate of Jiangsu Sanhe was protected by spraying zinc coating in 1966, and the coating has been in good condition for 28 years.