Analysis of Quality Problems in Production and Use of Coating Iron
The light industry standard QB/T 2763-2006 "coated tin (or chromium) thin steel plate" (abbreviated as coating iron) was released on August 19, 2006 by the National Development and Reform Commission and implemented on December 1, 2006. This standard is for the light industry standard QB/T 3773-1999 issued by the former State Light Industry Bureau (before the standard rectification in 1999 was the national standard GB8230-87) “Epoxy novolac coated tin (or chrome plated) steel sheet†Revision. In the revision process, we conducted investigations on various representative paint iron products. From the test results, we can see that the overall quality level of paint iron has been significantly improved, and most of the products meet the original standard QB/T 3773- Various technical specifications stipulated in 1999; According to the revised standard QB/T 2763-2006, acid resistance (except for chrome-plated iron due to unsuitable test solution), sulfur resistance, appearance, adhesion, coating film thickness The compliance rate is higher, which is 100%, 98%, 94%, 90%, and 92%, respectively, and the compliance rate of corrosion resistance and impact resistance is relatively low, which is 75% and 88% respectively. In recent years, the cause of corrosion of canned foods and beverage cans is related to the poor performance of coating iron such as corrosion resistance, impact resistance, and adhesion; of course, in addition to the quality of coating iron, improper use can also lead to can walls. Severe corrosion or perforation. The following analysis will be performed one by one through the cases in the test. First, quality problems caused by coating iron production 1, raw materials (1) Tinplate clearly specifies the iron requirements for canned foods and beverages in the coating iron standard QB/T 2763-2006. That is, tinplate should meet the requirements of GB/T 2520 Class I tinned steel sheet, and its surface energy is suitable for the entire sheet. Printing and coating. However, in order to reduce costs, companies sometimes use tinplates that are not suitable for foods or that do not correspond to the corrosion characteristics of foodstuffs. As a result, abnormal wall corrosion occurs. Canned sauce, coconut juice and other cans are not corrosive, but due to the poor quality of tinplate tinplate, the nominal tin plating amount of 2.8 g/m2 is only measured at 2.25 g/m2, and the exposed iron points are far more than normal iron. Although the thickness of the coating film is 6-8 g/m2, a large amount of corrosion occurs in the expansion ring of the top and bottom lids after several months of storage; for example, the amount of tin used for canning such as cherry juice, cherries, and aloe vera is 1.1 g/m. The m2 tinplate is very thin, and the minimum film thickness at the bottom cover and the can body is 3.3 g/m2 and 2.3 g/m2, respectively. The other two products are stored at 5-6 g/m2. In the near future, the cans of canned fruit cans were covered with densely corroded spots. Canned cherry juice and aloe cans were corroded and perforated in the cans several months later. The amount of iron dissolved exceeded 20 ppm. The surface condition of tinplate is poor, for example, the oxide film on the surface of tinplate is too thick (coated with low-chromium iron for canned mushrooms for one year), or the film is too thick and other factors affect the adhesion of coating film and the corrosion caused by processing is not resistant to corrosion. . The quality of canned food caused by the non-corresponding corrosion characteristics of tinplate and food, such as canned foods such as luncheon meat, pork ribs, chicken, sweet corn, broad beans, etc., all the indicators of coating iron used are in line with the original standards, but can Some severe sulphidization spots occurred after storage, even after sterilization. The reason is that the passivation degree of tinplate is low, and the surface chromium content is only about 1 mg/m2. This also proves that the surface of the tinplate is passivated (the 311 passivation should be used) Sulfur corrosion plays an important role. If this iron is used for loading corrosive acidic foods, there is no problem because the hydrated oxides of chromium and chromium in the passivation film dissolve quickly in the acidic solution and do not play a role in corrosion resistance. The above sulfidation corrosion cases also reflect the problem of poor correspondence between the original standard sulfur test method and the actual use effect (these paints can pass through the sulfur test), so the standard QB/T 2763-200 anti-sulfur test The method has been revised to use the internationally accepted method of cysteine ​​hydrochloride, which can effectively predict the anti-sulfur properties of coating iron. (2) The quality of coatings and solvents has a direct impact on the leveling properties, the compactness of the coating film, and even the hygiene issues of the food in the tank. When using paints with unqualified quality, the paint will not fully solidify even when the baking heat is increased under normal coating and baking conditions, and the coating film will be significantly whitened after being sterilized. This will cause the paint iron to fail the hygienic standard: epoxy phenolic The problem of excessive consumption of free phenol, free aldehyde, and potassium permanganate in the coating iron was solved after the coating formulation was improved. In addition, the above phenomenon may occur due to the self-aggregation of paints stored for too long. Any coating film that cannot be fully cured not only affects the porosity of the polymer coating film, but more importantly it is due to the organic substances that remain in the coating film that are not involved in the reaction and will migrate to the inside of the tank and cause food safety problems. The ratio of solvents and impurities in the paint can also cause quality problems in the iron coating. For example, the lack of high-boiling components in the mixed solvent affects the solubility of the coating and its wettability and adhesion to the tinplate surface. At the same time, the small bubbles formed by the large amount of low-boiling solvent in the coating during baking also affect the Microporous coating and impact resistance. Another example is the presence of obvious impurities in the coating iron, coating the gap in the discontinuity, installed a large sea of ​​jelly jelly corrosive phenomenon after several months; when the wax content in the coating as a lubricant is high, the film is dense The small bubbles affect the porosity and impact resistance of the coating, and the top and bottom covers are covered with corrosion spots after ketchup. The type of coating is not suitable for corrosion problems caused by the characteristics of foods. For example, full-open covers made of aluminum paste coating iron are used for acid foods with a PH value of less than 5, and the phenomenon of corrosion and perforation at the rivets and scribed lines is an example. Because the aluminum paste is easily soluble in the acidic solution, the corrosion resistance of the material is reduced, and the rivet and the engraved line itself are sites where the coating film is weak in protection and the stress is concentrated, so that concentrated corrosion is easily generated. 2, processing technology (1) The important role of the coating process on the uniformity of the coating film can not be ignored In order to save energy consumption, some companies often use unreasonable processing techniques, such as the use of corrosive acidic foods German paint iron, this should use double coating processing, due to physical coverage can effectively reduce the number of micro-holes, while coating The chemical binding force also enhances its denseness and adhesion. The use of a single layer of coating obtained by the thick coating, due to the coating used in the processing of high viscosity, poor leveling, the coating film must increase in micro-holes, so the tank wall after the tank has produced a large area of ​​diffuse corrosion spot or dense Corrosion point, serious coating film peeling phenomenon. This problem has occurred in canned tomato sauces, canned strawberries, and so on this year. Therefore, the coating process of coating iron must be based on the corrosive requirements of the installed products, and save costs under the premise of ensuring quality. The double-coated iron coating also has an effect on the processability of the coating film due to the improper coating thickness of each layer. If the coating is composed of aluminum paste and organosol, because the aluminum paste film is thick and soft, and the organosol film with good processability is thin, the coating iron is not rubbed and rubbed. The coating was damaged and there were micropores. Corrosion perforation occurred in the defect after the canned fish was canned. (2) Corrosion problems caused by insufficient baking. The coating does not reach enough heat to fully cross-link the reaction, not only affects the porosity of the coating, but also affects the flexibility of the coating. When the cover is covered, the coating cannot conform to the new geometry after the deformation of the substrate. Potential corrosion site behind the tank. Canned asparagus, beef and other cans used inadequately baked coating iron and soon after storage, significant iron sulfides appeared in the bottom cover expansion ring and countersunk head. The above corrosive conditions were in good agreement with the results of the dense corrosion point that occurred during the impact resistance test. After the paint was baked again, the impact resistance was improved. Judging and mastering the degree of baking can not be limited to the results of resistance to sterilization tests, or only consider the strength of adhesion. Often the thickness of the coating, adhesion, acid resistance through the iron coated ketchup will appear after the dense corrosion spots, because the paint has not yet reached the optimal degree of curing, if you increase the baking heat, curing and then fully, the corrosion resistance will There is a clear improvement. Therefore, for the coating of corrosive iron coating must be a comprehensive measure of its quality, especially the resistance to corrosion and impact resistance of the oven. The corrosion resistance index specified in QB/T 2763-2006 has a good practical significance. The correlation between the inspection results and corrosion of canned foods with acidic foods is good. Corrosion problems can easily occur after canning of materials that cannot withstand corrosion resistance. Therefore, it is very necessary to control the quality of coating iron in strict accordance with the baking process stipulated by the coating supplier and ensuring the normal operation of the drying room. In short, the quality of tinplate, paint, and solvent are closely related to the properties of coating iron. Although the organic polymer coating has the effect of blocking the contact between food and metal, the thickness of the coating film is limited, especially for the commonly used epoxy phenolic coatings, most of which are 5 to 10 microns. Excessive film thickness will weaken the adhesion. More or less, there will always be micropores in the coating film. When the cans are stored, the corrosive components in the food are inevitably brought into contact with the metal through the micropores of the coating film. Therefore, the coating quality and surface passivation of the tinplate must be taken into account in the processing of the iron coating, and the amount of tin plating should also be appropriate for the coating iron used for the installation of strong corrosive foods or pressure cookers. In the case of saving tin resources and material costs and using tinplate tin cans with low tin plating, coating iron producers must reduce the corrosion resistance by increasing the density of the coating film to reduce the amount of tin plating, according to qualified raw materials and Standard and reasonable processing technology is a necessary condition for ensuring the quality of coating iron. Second, corrosion caused by improper use of paint iron 1. Effect of Can Making on Iron Performance of Coatings The mechanical damage caused by improper adjustment of the canning equipment will weaken the anti-corrosion effect of the coating iron, and the corrosion of the tank wall will depend on the degree of damage of the coating film and the foods loaded. The characteristics. (1) Smeared mildly damaged, undamaged surface scratches on the tin layer will increase the micro-pores of the coating film, but have little effect on the corrosion of the tank wall of general corrosive products, especially for the can body is iron, top and bottom The sugar canned fruits and cans covered with iron coating can not be corroded by the tin layer at the scratches of the coating on the bottom cover due to the cathodic protection provided by the tin-dissolving reaction of the can body. (2) When the coating film is seriously scratched, and the coating iron is exposed to tin or exposed iron, the food is electrochemically reacted with the tin or iron through the damaged portion of the coating film to dissolve the tin or iron and the corrosion characteristics of the food. It was determined that the formation of massive diffuse corrosion stains caused the coating film to fall off, or iron dissolved deeply and perforate, both of which made food and beverage cans lose their commodity value. Tomato sauce and Red Bull beverage are products with strong tendency to dissolve tin, and have high requirements for uniformity and compactness of the coating film. Therefore, if there is severe abrasion, the contents will corrode with the tin where the coating film is incomplete and spread to the surroundings. As the iron dissolves, the adhesion decreases and the coating film falls off. Asparagus and luncheon meat are sulfur-containing foods that are prone to iron dissolution. When the top cover film is severely scratched and there is residual oxygen in the headspace, the hydrogen sulfide released by the protein during the sterilization of the can will be coated with the coating film. The iron in the defect produces iron sulfide. Although the aluminum paste coating iron has good sulfur resistance, because the coating film is soft and easy to be damaged during canning, it is necessary to pay special attention to the maintenance of canning equipment. When coated with chromium-plated iron is severely damaged, corrosion perforation often occurs in some corrosive acidic foods. For example, the coated chrome plated iron used for canned strawberry bottom cover has a low coating thickness of only 5-7 g/m2 and has serious scratches. As a result, the bottom cover is perforated several months after canning. This feature of pitting corrosion is determined by the electrochemical properties of the metal, which is an absolute anode due to the chromium of the iron chrome plating. Although the adhesion of the chrome-plated iron coating to the substrate is excellent, the chrome plating layer is Thin, mostly in the 50-80mg / m2, the number of microholes much more than tin-plated iron, once the scratches expose the metal, the probability of exposed iron is high, and the surface of the chromium has no cathodic protection of the iron at the micropores, so The corrosion of molten iron can be easily carried out and eventually lead to perforation. For the coating of chrome-plated iron, it is particularly important to control the thickness of the coating and its integrity. The determination of the coating thickness of the chrome-plated iron and the acid-resistance test method have new provisions in QB/T2763-2006. Severe mechanical damage can sometimes cause stress corrosion of cans and cracks. For example, the can body of a recently tested mushroom can cracked partially and the leakage is a typical result of stress corrosion. The main reason for this is that the scratches on the board are more and more serious, and the scratches are deep. After the canning, significant stress corrosion occurs on the pressed parts of the tin layer. One or more spots can be seen on the wall of the can. Straightly split. This corrosion problem has also occurred in foreign countries. According to their research reports, sulfur-containing and medium-corrosive canned foods coated with coated iron, when tinplate is sensitive to stress corrosion, residual stress during can processing, and free tin of tinplate When the amount is low or the plating of the chromium plated iron is not uniform and the coating film is poorly coated, stress corrosion and cracking are likely to occur. 2. Problems caused by solid tank processing (1) When the sealing machine is improperly adjusted or the mold is worn, the roll sealing operation in the hot state after the canning damages the top cover film, and the damage of the coating film at the full-open cover rivet scribing line is more timely than when the can is manufactured. more serious. Corrosion caused by damage to the outer coating of the curl and metal exposure. Since the cans are surrounded by oxygen during storage, moisture, salt, sulfur, or other corrosive components in the air are reacted with the iron in the curled film defects, and eventually a reddish-brown or orange-yellow iron hydroxide rust is generated. This kind of corrosion phenomenon is very common this year. Improper enveloping makes the full-opening engraved rivets significantly indented and deformed, and the cover height, burying degree become larger, the inner coating film is seriously damaged, and concentrated corrosion and even perforation problems occur at the defects after canning, this year in fruit. Canned vegetables have occurred. (2) Overcooling after sterilisation, insufficient drying, and beverage cans that are packaged outside the plastic film can cause localized corrosion of the outer walls due to residual moisture on the surface of the cans; residual cleaning after wiping the oil stains outside the tank after sterilization and cooling Agents cause corrosion problems at full-open cover engraved rivets. (3) The micro-leakage caused by bad seals, or the can's vacuum degree caused by the improper process of the solid can make the residual oxygen in the can too much and promote the corrosion of the inner wall of the can. (4) Food raw materials are not fresh (not processed in time), or there are residual pesticides in raw materials, etc. H2S leaches out excessively during sterilization, causing serious vulcanization spot problems, such as canned mushrooms, oxtail soup, steamed beef, etc. This type of corrosion problems; in addition to improper handling of raw materials, such as Bailing Mushroom, coconut fruit can be left in the excess of sulfur dioxide, resulting in significant sulfide corrosion. In summary, the causes of the abnormal corrosion caused by the canned food and beverage cans are various and sometimes caused by many factors. Therefore, in addition to selecting qualified raw materials suitable for food characteristics, reasonable processing techniques, and strict control of the quality of coating iron, the specification of empty cans and solid cans processing is equally important to prevent and mitigate tank wall corrosion. 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