Corrosion of The Inner Wall of Canned Acidic Food and Preventive Measures

Canned fruits in syrup, such as peaches and syrup oranges, are all acidic (PH≤4.6) canned foods. This type of can is relatively simple to sterilize, and only needs to be sterilized at normal pressure below 100℃. Canned meat, poultry and aquatic products are all low-acid (PH>4.6) canned foods, which require high-temperature sterilization. The shelf life of canned meat, poultry and aquatic products can generally reach more than two years, while the shelf life of canned fruits in syrup can only be one and a half years. The difference between the two is not a microbial problem, but mainly a series of chemical and electrochemical reactions between acidic foods and the inner wall of metal cans. If canned fruit in syrup is stored for too long, the heavy metal content in the contents is likely to exceed the standard (lead <1.0mg/Kg, copper <5.0mg/Kg, tin <200mg/Kg, arsenic <0.5mg/Kg), and the contents may also have a metallic odor (commonly known as iron smell), which will eventually cause hydrogen expansion and perforation and leakage, and lose edible value and commodity value. The corrosion phenomenon of acidic cans such as canned fruit in syrup is very complicated. Tin and iron are both negatively charged metals in the electrokinetic series. Their negative charges are stronger than hydrogen, and they can replace hydrogen in acidic solutions. The potential relationship between tin and iron will change because they are adjacent in the standard electrokinetic series. Therefore, if the objective conditions change, it is easier to cause the potential relationship between tin and iron to change accordingly.

In canned fruit, if there is no oxygen, the corrosion of tin is very small and has no significant effect. If there is air or an oxidant, depolarization occurs, with tin as the anode and iron as the cathode. The tin layer begins to corrode and dissolve, gradually exposing the base iron. The iron becomes the anode, and the surface tin layer remains intact, while the iron at the hole corrodes and the hole continues to develop.

In general, the acid corrosion of the tin-plated thin steel plate on the inner wall of canned food is different from that in the atmospheric environment. The corrosion of iron is suppressed, but the corrosion of tin is promoted. The reasons can be summarized as three points:

1. When air, as an oxidant, does not exist, the tin layer is not easy to discharge, and only the steel base surface discharges, resulting in the mutual conversion of the tin and iron electrodes;

2. The exposed iron area is small and the tin area is large, and the concentration of ions dissolved in the solution is very different, which also becomes the reason for the mutual conversion of the two poles, making tin the anode and iron the cathode;

3. The higher the acidity, the greater the positive charge on the tin surface. In high acid, the amount of tin dissolved is large and the amount of iron dissolved is small. However, when the acidity is low, the amount of tin dissolved is relatively small, while the amount of iron dissolved increases.

Ⅰ. The corrosion process of the inner wall of tin-plated thin steel plate by acidic canned fruit can be roughly divided into three stages:

a. The stage where the tin layer fully covers the steel base

In this stage, the residual oxygen in the can has been consumed as an oxidant when the contents are oxidized or the can wall is corroded. Therefore, in this stage, the steel base of the tin-plated thin steel plate becomes the cathode, and the tin becomes the anode. The tin dissolves into tin ions, which is exactly the opposite of the situation when there is a large amount of oxygen. The steel base exposed through the micropores does not dissolve, and because the exposed iron surface is very small, the amount of hydrogen produced is very small, which has almost no effect on the corrosion process. Therefore, in this stage, it can be said that the exposed steel base is protected by sacrificing tin (anode), so only the tin layer is corroded alone.

From the perspective of tin dissolution, the factors that determine the length of this stage are mainly the thickness and coverage density of the tin layer, the role of depolarizers and corrosion inhibitors in the canned food, and the situation of the tin itself, etc. The latter two are important factors affecting the dissolution rate of tin. The corrosiveness of the steel base has no effect on the dissolution of tin. The storage life of canned food is mainly guaranteed by extending this stage.

b. The stage where the exposed iron area of ​​tin-plated thin steel sheet expands to a considerable extent

In this stage, tin dissolves rapidly, and the exposed area of ​​the steel base has expanded to a considerable extent, so it is also a process in which tin and iron corrode simultaneously. The dissolution of tin is affected by the properties of the steel base, and a large amount of hydrogen is produced. By the end of this stage, before entering the third stage, it has actually become a waste product, and even hydrogen bulges the can, and sometimes there is occasional perforation.

The main factors that determine the length of this stage are the corrosion resistance of the steel base, the role of the corrosion inhibitor in the canned food, and various factors that affect the potential relationship between tin and iron.

c. The stage in which the tin layer on the tin-plated thin steel sheet is completely dissolved

In this stage, only the difference in the properties of the steel base remains. The storage of canned food has ended here and is no longer determined by the length of this stage.

Ⅱ. The corrosion conditions and procedures of the inner wall of the tin-plated steel plate container by acidic food cans are not completely consistent. There are several phenomena as follows:

1. Uniform corrosion: In acidic fruit cans and juice cans, the tin layer on the inner wall of the tin-plated steel plate container will be fully and evenly dissolved under the corrosion of acidic food, so that the tin layer grains in the can wall are all exposed, and the surface of the tin-plated steel plate is fish-scale-like and uniformly corroded. The tin content of the contents increases. If the storage time is too long, the corrosion continues to develop, the tin layer peels off on a large area, the steel base is exposed, the amount of dissolved tin increases sharply, the food in the contents has a metallic taste, and a large amount of hydrogen is produced, causing hydrogen expansion, and in severe cases, expansion and cracking.

2. Local corrosion: At the junction of the top gap and the liquid surface of the canned fruit in syrup, due to the effect of residual oxygen in the top gap, the can wall is corroded, forming a dark brown corrosion circle (commonly known as an oxidation circle).

3. Concentrated corrosion: A limited area of ​​iron dissolution occurs on the inner wall of the can. Such as pores, pitting, black spots, and in severe cases, perforations and leaks appear on the can wall, causing the food in the can to be contaminated, deteriorated, and corrupted. There is a lot of oxygen in the can or some fruit tissues contain gas, and the gas is not removed by vacuuming or pre-cooking before canning.

4. Abnormal detinning corrosion: canned foods such as orange juice and tomato products are all detinning canned foods. Because they contain special corrosion factors such as nitrate or nitrite, they directly react chemically when they come into contact with the inner wall of the can. In a short period of time (2 to 3 months), a large area of ​​detinning occurs, and the tin content in the contents exceeds the standard (200 mg/Kg). During the detinning stage, the vacuum degree decreases very slowly, and the appearance, rod beating inspection, and vacuum degree measurement are all normal. However, after the detinning phenomenon ends, hydrogen expansion will occur rapidly.

5. Bimetallic corrosion: In order to truly reflect the requirements of convenient food, canned food must solve the long-standing problem of "canned food is delicious but difficult to open". Metal cans must use easy-open lids. Canned food is difficult for consumers to open, which is also a factor that makes it difficult to sell canned food in China. Therefore, it is more urgent to use pull-open lids for canned food to be sold domestically than to export canned food. The opening of pull-open lids is better with aluminum alloy than with tinned thin steel sheets. Therefore, a few years ago, a canned food factory in Guangdong produced canned tomato juice. The can body and bottom were made of tinned thin steel sheets, and the can lid was made of aluminum alloy pull-open lids. 0.5% salt (containing 303 mg/Kg of chlorine) was added to the contents. After being stored for several months, leaks appeared at the rivets and the line, and the production was forced to stop. Tinned thin steel sheets are used for the can body and bottom. If the can lid is made of aluminum alloy pull-open lids, micro batteries are formed after the food is canned, and a double gold reaction occurs. Aluminum is the anode and tin is the cathode. When the cathode area is larger than the anode area, local deep pitting occurs at the anode, and sometimes even perforation. According to Taiwan literature a few years ago, coconut water beverage cans were popular in Taiwan at that time, and aluminum alloy pull-top lids were used. Since the chloride ion concentration in the contents reached 440~1492 mg/Kg PH=4.4~4.6, mainly malic acid. In the early stage of canning, some corrosive components (malic acid, chloride ions) penetrated the coating and carried out electrochemical reactions at the damaged part of the coating. At this time, the coating was the cathode, and the tin or iron exposed at the damaged part of the coating was the anode, until a considerable area of ​​tin and iron was exposed, and the aluminum exposed by the aluminum pull-top lid due to the damage of the coating was connected to form a local battery, and a bimetallic reaction was carried out. Aluminum was the anode and corroded and dissolved. When the cathode area became larger and larger (the larger the exposed area of ​​tin and iron), the corrosion rate became faster. In addition, the presence of chloride (cl) ions in the contents accelerated the corrosion reaction, causing the pull-top lid to be perforated. In addition, it was reported that some high-acid food canned beverages such as carambola juice, sour plum juice, and olive juice had also experienced such phenomena.Therefore, for some high-acid foods, those containing more than 100 mg/kg of chloride ions, including some meat, poultry, aquatic products, vegetables and other condiments (due to the addition of salt), the can body and bottom are made of tin-plated thin steel plates, and the can lid and pull-off cover cannot be made of aluminum alloy, but should be made of tin-plated thin steel plate.

6. Others: Stress corrosion will also occur at the can lid expansion ring, the rivets of the pull-off cover, the scoring and the can body reinforcement ribs.

Ⅲ. Corrosion of the inner wall of the coating can.

Now the canned fruit in syrup generally uses plain iron (tin-plated thin steel plate without coating) for the can body, and the can lid and bottom are all made of coated iron. However, some varieties have high acidity and contain anthocyanins (anthocyanin pigments), such as canned bayberry, strawberry, kumquat, mango, etc., which should be acid-resistant and fully coated.

The coating on the coating can should be fully covered on the surface of the tin-plated steel sheet. If the coating on the coating iron can be uniform, complete, dense, without holes and damage, no corrosion will occur. It should be said that good quality coating is the ideal measure to prevent corrosion of the inner wall of the can. In fact, there are problems in the coating construction and canning operation technology. The coating layer has holes and damage, and it is difficult to be perfect.

Where the coating can film layer is damaged, only the local tin layer is exposed, and the corrosion proceeds horizontally under the coating film, causing corrosion under the coating. The cathodic protection of tin to iron is reduced to a minimum. If it continues to develop, the coating film can be seen bubbling and floating, and the coating film falls off. Unlike the reaction on a large area of ​​tin layer in the plain iron can, it is concentrated in a small area of ​​galvanic formation, and the corrosion is deep until perforation. Sometimes the situation is much more serious than that of the plain iron can.

Ⅳ. Various factors affecting the corrosion of the inner wall of the can

The corrosion situation and degree of the can are essentially the result of the contradiction and interaction between the corrosive components in the raw and auxiliary materials of food and the tin-plated steel sheet. Depolarizers and corrosion inhibitors in food raw materials and auxiliary materials, as well as external factors such as canned food processing technology and storage conditions, promote or delay the corrosion of the inner wall of the can. The characteristics of agricultural production (origin of raw materials, maturity, harvest time, etc.) and environmental issues will also be reflected in the corrosion process of cans.

The corrosion factors of various foods, auxiliary materials, and spices that cause corrosion to the inner wall of cans include: oxygen, organic acids, dehydroascorbic acid, low methoxymethyl esters, nitrate ions, anthocyanin pigments, caramel, soy sauce, salt, sulfur and sulfide, copper, trimethylamine oxide, etc.

Ⅴ. Precautions to be taken to prevent corrosion during canning

(I) Quality requirements for tinplate

Before canning, you must first select tinplate with good corrosion resistance. The steel base, tin-iron alloy layer, tin layer, passivation film, etc. of the tinplate have different degrees of relationship with the corrosion resistance of the tinplate. The corrosion resistance of the tinplate is usually comprehensively evaluated by measuring the following items and indicators:

1. Iron dissolution value (ISV): The better the continuity of the tinplate, the fewer exposed iron points, the less iron dissolution when in contact with acid, and the better the corrosion resistance. Tinplate with high corrosion resistance requires ISV≤20μ g.

2. Tin layer grain size (TCS): The size of the tin layer crystal. Tinplate with large grains has good corrosion resistance. The requirement is not less than grade 9, that is, the grain diameter is not less than 1.6μ g.

3. Alloy tin coupling test (ATC): It is used to judge the continuity of tin-plated steel sheets and the properties of the steel base surface exposed through the pores of the alloy layer. The lower the ATC value, the better the corrosion resistance to acidic foods and the longer the shelf life of canned food. The requirement is ≤0.05μ A/cm2.

4. Acid leaching hysteresis value (PL): The surface of the steel substrate is clean, with few impurities and uniformity, and the acid leaching hysteresis value is small, and the corrosion resistance is good. The requirement is ≤10S.

5. Passivation film: The passivation film generated by chromate treatment, the higher the chromium content, the better the corrosion resistance. The passivation film is easy to fall off in acidic fruit cans with a pH of ≤5. If cathode treatment is performed, the passivation film can be firmly attached to the tin-plated steel sheet.

6. Oxide film: The tin layer of the tin-plated steel sheet is oxidized to form an oxide film (Sno2 or Sno), which is not conducive to corrosion resistance. The corrosion resistance depends on the amount of Sno2 and Sno.

7. Surface defects. Due to improper operation during the production of tin-plated steel sheets, defects such as pits, folds, missing corners, edge cracks and solvent spots may occur on the surface. These are not allowed to exist.

8. The original steel type of tin-plated steel sheets should be L-type, which has the advantages of less impurities and strong corrosion resistance. According to reports, two types of steel have appeared recently, K-type plates and J-type plates, which are made into tin-plated steel sheets. The tin plating amount is not less than 5.6g/m2, and it has good corrosion resistance to certain foods with strong electrochemical detinning effect.

(II) Tin layer scratches during canning and preventive measures

The tin layer is scratched during the canning process, and in severe cases, the alloy layer may be damaged. It is an important factor that promotes chemical and electrochemical reactions between acidic foods and tin and iron, and accelerates the corrosion of the inner wall of the can. Therefore, every process of canning must be operated carefully. The reasons and prevention methods for scratching the tin layer in the main process of can making are described as follows:

1. The adjustment and control of the pressing plate and positioning plate of the cutting machine should not be too tight, otherwise it will cause the can body strip-shaped drawing scratches.
2. The burrs of the can body are too large, and the bending and rounding devices are not adjusted accurately, which can easily cause scratches on the can body. It should be adjusted according to different can body heights, or replaced with round rollers.
3. The can body conveying track is worn, and dust and impurities will also cause scratches on the can body.
4. Improper adjustment of the sizing gauge, too tight, and inflexible guide wheels will cause scratches.
5. The can delivery track and can distributor are worn or adjusted too tightly, which will cause scratches during the can delivery process.
6. The flanging mold is worn, especially the impact flanging machine, the flanging mold is not standard, the flanging curve is not smooth and has a segmented slope, which will cause flanging mouth scratches. The flanging spinning wheel of the spinning flanging machine is not flexible and not calibrated properly, which can also cause scratches on the flanging edge.
7. Scratches on the canning edge

• The canning accuracy of the canning machine is poor

• The canning pressure head and roller are not standard, and the pressure head and roller are not selected according to the different thickness, hardness and can type diameter of the tinplate.

• The curve of the canning roller is not smooth or has been worn.

• The canning pressure head and roller do not match and are improperly adjusted.

• The can lid (bottom) is not properly dropped and does not match the canning groove shape.

VI. Measures that canned food factories should take to prevent corrosion of the inner wall of cans

In the current commodity economy, where products are generally in oversupply, enterprises should actively study to improve product quality and extend shelf life in order to be competitive in both domestic and foreign markets. When processing canned food, canned food factories should first study and test the varieties to be processed in detail, what factors are there that cause corrosion to the can wall, and explain to the canning factory and put forward requirements for the quality of sealed containers. Canned food factories should study the following aspects in terms of processing technology:

1. The amount of residual oxygen in the can, the less the better. There are more than 10 factors that cause corrosion to the inner wall of cans, but the main cause of corrosion to the inner wall of cans is residual oxygen in the can, which is a common problem. Therefore, eliminating the residual oxygen in the can is not only a physical or microbiological need for processing canned food, but also very necessary from the perspective of preventing corrosion of the inner wall of the can. Oxygen is a strong oxidant for metals. The oxygen in the can acts as a cathode deoxidant in an acidic medium, showing a strong oxidizing effect on tin. There is an obvious linear relationship between the amount of tin dissolved and the concentration of oxygen. Oxygen is consumed when tin is dissolved. When all oxygen is consumed, the amount of tin dissolved is greatly reduced. According to calculations, 1ml of oxygen in the can can dissolve about 10.6mg of tin; 1ml of oxygen can dissolve 4.9mg of iron. When there is more than 3ml of oxygen in the can, the corrosion of the sugar solution interface of the canned fruit in syrup, i.e., the oxidation circle, is obviously formed. Nitrate ions are strong corrosion factors, but if there is no oxygen in the can, it will not cause the dissolution of tin. In the presence of oxygen, it will cause abnormal tin stripping, dissolving tin ions, turning nitrates into nitrites, part of which is reduced to nitrogen, and the other part, due to the action of tin ions, returns to nitric acid. At this time, oxygen plays a role in promoting the entire stripping reaction. Therefore, when nitrate ions are present, because oxygen is the promoter of the tin dissolution reaction, the tin dissolution reaction can only occur when oxygen is present, and nitrite ions are the promoter of the tin dissolution reaction under anaerobic conditions. When the residual oxygen in the can is exhausted, nitrite ions can replace oxygen. Therefore, if nitrite ions exist after oxygen is exhausted, nitrate ions will still promote the dissolution of tin. The residual oxygen in the can affects the color and flavor of the contents. There are two types of browning of fruits in the can: enzymatic browning and non-enzymatic browning, and non-enzymatic browning is absolutely related to the presence of oxygen. The ferrous ions dissolved by the corrosion of the inner wall of the can are affected by oxygen, changing from 2-valent iron to 3-valent iron, and reacting with phenol to blacken, which significantly reduces the commodity value of food, such as the discoloration of coffee and tea beverages. Apples, pears, and pineapples contain more air inside the fruit tissues, so it is best to use vacuum treatment. Some fruit raw materials are pre-cooked before canning, which can reduce the air content in the raw material tissues.

2. Use heating exhaust to appropriately increase the vacuum degree in the can. The sugar water injected into the can should be boiled to drive out sulfur dioxide and air in the sugar.

3. When canning, prevent the top gap from being too large, and the sugar solution must be filled.

4. Highly mature raw materials should be used for canning. Raw materials should be thoroughly cleaned before processing to remove attached pesticides and other chemicals. Fruits that have been peeled or coated with acid or alkali (oranges, peaches, etc.) or treated with chemical color protection must be thoroughly rinsed to remove attached chemicals.

5. Control the appropriate sterilization temperature and time, and quickly cool to 30~40℃ after sterilization to minimize the heating time of canned food during the processing process. Some canned varieties are packed after sealing, with the bottom of the can facing up, and the can lid facing up after sterilization. The can is turned upside down many times to reduce the problem of concentrated corrosion at the sugar solution interface.

6. For raw materials such as pears, peaches, and plums, varieties with lighter anthocyanin pigments should be selected. For fruits with high anthocyanin pigments such as bayberry and strawberry, acid-resistant full-coating cans should be used.

7. Minimize the addition of acid without affecting the quality of the contents. For products that need to add ascorbic acid, try to reduce the amount to prevent ascorbic acid from being heated for too long during processing, which will cause ascorbic acid to become dehydroascorbic acid (corrosive factor).

8. Raw materials should be selected from varieties with less nitrate and nitrite ions, and the nitrate should not exceed 3mg/kg. Strictly control the nitrate and nitrite ion content in processing water, especially the quality of water added to cans, and the nitrate should not exceed 1mg/kg.

9. The storage temperature of canned products should not be too high, and the warehouse should be ventilated, cool and dry.

10. To prevent corrosion of the inner wall of the can, corrosion inhibitors can be added within the scope specified by the additive hygiene regulations. For example, adding colloidal substances, animal glue, pectin, etc., adding 0.2~0.4% animal glue to high-acid canned fruit (PH=2.93~3.76) can extend the shelf life of the can. There are also reports that the effect is not obvious. For example, stannous ions can inhibit the corrosion of iron, and the effect is very obvious. When the stannous ion content is above 10mg/kg, it can protect the steel base. When using plain iron cans, under the action of sterilization heat, sufficient stannous ions will be dissolved in the acidic juice. This is an important factor that canned food can still be stored for a long time even though the inner wall of the can is uniformly corroded by acid.