Corrosion Engineering Corrosion in Metal Food Containers.

(GAMRY Instruments) (GAMRY Instruments)

Corrosion Engineering

Corrosion in Metal Food Containers

Abdullah Jan | 17452656 |

1. Table of Contents

1    Introduction        2

1.1        background information        2

1.2        corrosion        2

1.3        sample        2

2        case study of corroded tin coated steel can        3

2.1        conditions        3

2.2        observation of the corroded metal        3

2.3        Mechanism        5

2.4        other factors that encourage corrosion        7

2.4.1        Salt water (Sodium Chloride)        7

2.4.2        Tin coating        7

2.4.3        Iron(III) oxide deposits        7

3        electrochemical technique        7

3.1        Linear polarization Resistance        7

3.2        Lpr test cell        7

3.3        lpr advantages        8

3.4        applications        8

3.5        Technique        8

4        conclusion        10

4.1        Verdict        10

4.2        prevention        10

5        References        11

1    Introduction

1. background information

Metal food cans, commonly known as tin cans, have been in practice for decades to preserve food. These cans contrary to their name are not entirely made from tin as it is rare and the manufacturing of food containers in such a large amount would be expensive. These containers are made of either low carbon steel or aluminium and then coated with tin due to its high corrosion resistance to acids that foods normally produce (Lacoma, 2018). Steel is an alloy made from carbon and iron, both the elements occur naturally which contributes towards the low cost. Iron is completely recyclable which makes it easier to be used repeatedly in the industry making it sustainable. Despite noble-coating the metal, food containers are still a victim of corrosion. Corrosion is one the biggest concerns in the engineering design world but with careful designing, research & development along with the help of electrochemical techniques the factor of corrosion may not entirely be solved but it can be controlled and slowed down to make the design more efficient and avoid failure.

1. corrosion[pic 1]

The ‘preferred’ state for metals is their oxidised state and they achieve this by an electrochemical process called corrosion. Corrosion results in degradation of manufactured metals which results in mass loss (Kain Escobar, 2014). An electrochemical process, in terms of corrosion, is when there is oxidation and reduction taking place simultaneously, also known as a redox reaction, this forms a Galvanic cell. In a Galvanic cell, oxidation causes the anode to give out metal ions, the electrons pass through the circuit as shown in  Figure 1 and into the cathode causing reduction (Maqsudi, n.d.).

In the sample can, corrosion can occur in two sides, the outer side of the can or the inner. In the case evaluated, the inner side is studied as the can was filled with salt water.

1. sample

For this case, a low-carbon steel food container electro-coated with tin as a protective layer is studied. In the earlier years, steel sheets were used to manufacture food cans but due to high rate of corrosion and contamination of food, protective layers were introduced. The food container was filled with salt water, sodium chloride, and left at room temperature. After 45 days, the can was opened to observe any corrosion. The most suitable and efficient electrochemical technique is recommended to monitor the observed corrosion based on its nature.

1. case study of corroded tin coated steel can        

1. conditions[pic 2]

For aqueous corrosion to occur there needs to be an anode where oxidation reactions will take place, a cathode where reduction reactions will take place and an electrolyte that will act as a corrosive medium. Water is a weak electrolyte as the OH-  and H+ ions do not dissociate fully to conduct the electrons, an electrolyte with dissociated electrons such as aqueous sodium chloride makes a good electrolyte. There should be a constant supply of oxygen as in absence of oxygen there will be no corrosion (Gordonengland, n.d.).

1. observation of the corroded metal[pic 3]

Corrosion was observed at the mouth of the steel container, the inside of the lid, the inner walls of the container and the joint where the steel sheet sealed to give it a cylindrical shape. No corrosion was observed on the outer side of the container possibly because the Tin coating was still intact and there was not enough moisture to initiate a galvanic reaction.

• As figure 2, 3 and 4 show there is significant corrosion(rust) on the inside of the lid. This is due to mechanical stress put on the lid while opening and closing the container. The wear & tear resulted in a damaged protective tin layer which exposed the steel underneath to moisture and salt water splashed while moving the container.                                                           

[pic 4]

[pic 5]

• Figure 5 and 6 show the most visible corrosion on the seal. Since the container was filled with salt water the inside had the most contact. The seal is most effected probably because that is where the steel overlaps impeding oxygen exposure. Some areas ended up in corroding to such an extent that the metal sheet had holes in it. These holes are not visible in these pictures.

• Figure 7 shows small areas on the surface of the metal sheet that are corroded. The corrosion is not major but it does respond to the mechanism of pitting due to chemical reactions inside the container.[pic 6][pic 7]

1. Mechanism

        A common type of corrosion in steel is pitting. The corroding part or pit of the metal acts as an anode, transfers electrons through the metal and oxidizes with the help of a depolarizer, commonly oxygen, the rest of the metal becomes a cathode. For aqueous corrosion to take place, there must be an electrolyte present, in this case – the salt solution (Lower, 2017). Metal ions flow into the electrolyte, as illustrated in figure 8A, from the anode and electrons travel through the metal towards the cathode (non-corroded area of the metal) from within the metal structure and get further involved in cathodic reactions to form Rust (Iron(III) oxide) as illustrated in figure 8B.[pic 8][pic 9]

To prevent this type of corrosion of steel, a protective layer of another metal or metal oxide is applied on the main metal, which in this case is steel coated with a film of tin. The noble-coating of tin (figure 9), forms a barrier between the steel and the environment, hence stopping corrosion. In reality, this film alone is not enough to stop corrosion. When the tin layer on steel is damaged, the exposed steel underneath is exposed to salt water. Oxygen diffuses into the electrolyte from the atmosphere and creates an oxygen-concentration gradient where the pit has relatively less oxygen due to restricted access and the rest of the undamaged metal surface has more oxygen (Figure 10). Due to the scarcity of oxygen at the pit(s) that area becomes anodic and the rest of the metal, where oxygen is ample, acts as a cathode. This completes the process path and satisfies all the conditions of a galvanic cell. There can be multiple pits all over the surface of the metal, of different sizes and shapes, figure 11 illustrates this. The order of reactions is as follows (Helmenstine, 2018) (Kain Escobar, 2014) :[pic 10]