At present, the commercial cold-rolled carbon steel coil has been on the market for more than one year, and the objection to coil corrosion accounts for about 60% of quality complaints. According to the corrosion of a large number of packaged steel coils and more than one year's rectification practice, three major factors leading to the corrosion of commercial cold-rolled carbon steel coils are obtained: (1) the surface and appearance quality of finished steel coils; (2) packaging materials and packaging technology; (3) The influence of circulation. Only by analyzing the corrosion mechanism and influencing factors can we take corresponding measures to reduce the probability of corrosion.
I. Corrosion mechanism
For carbon steel cold-rolled coil, the main corrosion mechanism is electrochemical corrosion, which can be divided into hydrogen evolution corrosion and oxygen absorption corrosion because of the different PH value of the medium in contact with the surface of strip steel.
1. If the air contains more acidic gases, such as CO2, SO2 and NO2, the metal is exposed to humid air, which is prone to hydrogen evolution corrosion, as shown in figure 1.
The corrosion process is as follows:
Anode (Fe): Fe): Fe→Fe2++2e-
Fe2++2H2O→Fe(OH)2+2H+
Cathode (impurity): 2H+ +2e-→H2
Battery reaction: Fe+2H2O→Fe(OH)2+H2
Figure 1 Schematic Diagram of Electrochemical Corrosion
2. When the water film adsorbed on the metal surface is weakly acidic, oxygen absorption corrosion will occur due to the action of dissolved oxygen, as shown in figure 1.
The corrosion process is as follows:
Anode (Fe): Fe): Fe→Fe2++2e-
Cathode (impurity): O2+2H2O+4e-→4(OH)-
Battery reaction: 2Fe+2H2O+O2→2Fe(OH)2
Second, the influence of product surface quality and appearance quality
1. When coiling, the steel coil has poor flash and trimming.
These two kinds of edge defects will mainly damage the rust-proof paper and plastic sheets used in packaging. When the antirust paper and plastic board are punctured and cut, the gas-phase antirust agent and antirust oil will diffuse and volatilize, and the external acidic atmosphere, moisture, oxygen and water will invade the interlayer surface of the steel coil, thus causing electrochemical corrosion of the metal surface and forming corrosion. For the overflow, in order not to cut the rust-proof paper and plastic cloth, paper corner protectors must be placed at the end of the steel coil first, and then the outer ring retainer should be added after the packaging is completed; For the serrated edge, polish the serrated edge with a polishing machine, and then apply a proper amount of oil locally.
2. Residual leveling liquid on the surface of steel coil
In the production of temper mill, if the temper liquid cannot be thoroughly cleaned due to the defects of the cleaning system, the residual temper liquid will adhere to the tape layer. The leveling liquid is composed of 95% water and 5% detergent, and the remaining leveling liquid between steel coils will lead to local corrosion defects. Therefore, good air purging effect is the key to ensure that leveling liquid does not remain on the board surface.
3. The oil on the board surface is uneven
Cold-rolled surface will be coated with a layer of oil before crimping, and antirust oil is only a transitional protection. High-quality antirust oil can make metal rust-proof for up to two years. The amount of oil coated on the surface of wood board will affect the antirust effect. Excessive oiling will cause the antirust paper to be soaked by excessive antirust oil, which will lead to the failure of the antirust paper. Mild oiling will lead to the defects of antirust oil film, and the local area of the board will not be protected and local corrosion will occur. Therefore, proper oiling is very important, and the adjustment of oiling quantity should be optimized according to the feedback information from customers.
4. Bundle layout
At present, most iron and steel enterprises only use circumferential banding tapes for bare coils after cold rolling, but for coils larger than 2.0mm, lead and the whole coil form a certain separation angle at the tangent point, which is easy to be cut off or burst when wrapping antirust paper and plastic sheets, and the appearance after packaging is irregular and prone to bulging.
Three, packaging materials and packaging operation technology
Excellent packaging materials are the main guarantee that steel coils are not affected by unfavorable factors such as external oxygen, acid and humidity during circulation. Especially for products sent to southeast coastal areas and humid climate areas, the most basic packaging needs to adopt four layers of gas-phase rust-proof oil paper, plastic sheets, outer protective plates and inner and outer retainers.
1, the influence of vapor-phase antirust paper
Vapor-phase rust-proof paper is a composite material in which paper pulp is coated on plastic woven cloth, and then corrosion inhibitor dissolved in water or solvent is coated or sprayed on neutral packaging paper. Generally, the coating amount is 5 ~ 10g/m2.
The antirust principle of vapor-phase antirust paper is mainly to form a thin protective layer on the metal surface through the gradual volatilization and diffusion of corrosion inhibitor, so as to isolate moisture, carbon dioxide, hydrogen sulfide, sulfur dioxide and other gases in the air, inhibit corrosion and achieve antirust effect.
Vapor-phase antirust paper generally uses pressure-sensitive tape to close the lap or seam, wraps the steel coil in the closed environment formed by antirust paper, and uses vapor-phase antirust agent to separate the steel coil from the outside to delay corrosion. Therefore, the antirust performance of vapor-phase antirust paper depends on the sealing property of the wrapping layer formed by antirust paper. In the process of packaging operation, whether the antirust paper is laid symmetrically and smoothly, whether the end face and eye overlap with the inner waterproof cardboard, and whether the adhesive seal of the pressure-sensitive adhesive tape at the joint is effective will directly affect the antirust performance of the packaging.
2, the influence of plastic sheeting
The main function of plastic paper is to block the entry of moisture and moisture, and to seal the gas-phase antirust agent diffused from the antirust paper for the second time, so the antirust effect of plastic paper depends on its permeability. At present, the widely used packaging materials are plastic films and composite films, which cannot be completely blocked due to the diffusion of water vapor and gas molecules. So the quality of plastic board is the key to rust prevention. The thickness is generally not less than 0. 1mm, and the barrier or permeability of the plastic sheet can be judged by checking whether there are defects such as air holes and bubbles in the plastic sheet through light transmission. Secondly, the plastic sheet should have a certain strength to ensure that it will not be damaged or reduce the degree of damage under the condition of being pulled and hit by external forces.
It is best to wrap plastic sheets through the core. One end of the plastic cloth passes through the coil hole of the steel coil, and the end face of the steel coil is bonded and sealed with adhesive tape. Its process is the same as rust-proof paper, and it must be sealed and bonded firmly at the joints, and no air holes can be left.
3, outsourcing guard plate
At present, different steel mills mostly use plastic guard plates or waste steel plates on production lines as outer cladding plates. Plastic fender is the most economical packaging material at present, which is easy to form, recyclable and low cost, and has become a trend of commodity packaging at present. As an outsourcing version, PE board is generally used as the material, which requires certain barrier performance and impact resistance. High-strength polyester PET material is selected for high-grade packaging.
The guard plate must be made of a whole plate, which is undamaged, pressure-sensitive and completely bonded and sealed to prevent rain and moisture from entering. It is best to be in the horizontal position of the steel coil (that is, at 3 o'clock or 9 o'clock, such as the hour scale of the clock plate), so as to avoid the rain from flowing backwards.
4. Internal and external fixators
Generally speaking, the iron retainer plays the role of sealing the end cover and the outer cover. Galvanized medium carbon steel is generally used, and low-carbon cold-rolled galvanized sheet with thickness of 1.8 mm ~ 2.0 mm can also be used. The included angle between the two vertical surfaces of the cage is preferably controlled at 901to keep close contact with the guard plate and prevent rainwater from entering. For the eye-rolling iron retainer, it is not only a special protection for the edge of steel coil, but also a load-bearing protection during hoisting. Its strength and thickness must be guaranteed. If the strength is too low and the thickness is too thin, the strap will be easily deformed after tightening. In the process of hoisting, there will be a gap between the inner roller retainer and the end guard plate, which will inevitably lead to water ingress in the packaging layer. Therefore, the head-tail joint of the rolling eye iron retainer is controlled at 4.
Four, the circulation of the damage to the packaging
1. Damage caused by hoisting during operation and circulation.
In the process of packaging circulation, the more transit links and loading and unloading times, the greater the probability of packaging damage. The transportation of steel coils mainly depends on trains. From the loading quality of finished products and the appearance of stock coils fed back after sale, the damage of packaging is mainly caused by the friction, scraping and collision of the outer guard plate, followed by the warping deformation of the grommet, the fracture and looseness of the steel belt, and the scraping and deformation of the outer iron retainer during lifting. From the circulation point of view, in order to reduce the probability of corrosion, it is necessary to start with the material of the outer guard plate and adopt the material with high impact resistance (that is, high brittleness). In order to reduce the damage of the outer guard plate during operation and circulation, some manufacturers use cold-rolled scraps with high strength as the outer guard plate.
In the process of circulation, the technical level of steel coil hoisting operators and the environment of hoisting site are the main factors affecting packaging damage. All departments and links need to improve their operation skills, strengthen their sense of responsibility and operate carefully to reduce the corrosion of steel coils.
2. Influence of storage environment and storage period
Although the rust-proof period of rust-proof oil and rust-proof paper between steel coils can reach 2 years, the rust-proof performance of steel coils can only be guaranteed if the relative humidity in the air in the storage environment reaches a certain balance and the moisture absorption is equal to the evaporation. However, according to customer feedback, most of the outer packaging was damaged during circulation, which greatly shortened the validity period of rust prevention. In addition, the rainy and snowy season in Xia Dong and the humid and rainy climate in the south are the high-frequency periods of quality objection of corrosion defects. Therefore, it is a way to coordinate cost and quality to classify packaging quality according to customers' storage conditions, sales destinations and service time of end customers.
Through the above analysis, as well as the corresponding measures taken to improve packaging quality and strengthen quality management monitoring, from the middle of September 20 10 to now, the number of objections to corrosion quality of carbon steel cold-rolled commercial coils has been greatly reduced.
refer to
[1] Song Pannian, Dai,. Packaging Technology Beijing: Printing Industry Press, March 2007.
[2] Zhou, Rust Prevention Technology [M]. Beijing: Chemical Industry Press, 1988.
[3] Yan Zheng. Rust prevention of metal products [M]. Beijing: Commercial Press, 1986.