Dairy technology equipment

Author: Chief Editor: Wu Jianxin
Publisher:
Publish Date: 2004-09-01
Features:
Fragment: p;
1. Composition of Stainless Steel
There are many types of stainless steel, and the commonly used ones in the dairy industry are as follows.
(1) 0Cr13, 1Cr13, 2Cr13
These materials have certain toughness, so they are generally used to manufacture containers for powders (e.g., milk powder), such as linings, exteriors, and general bolts and nuts.
(2) 3Cr13, 4Cr13
These materials have certain hardness but poor weldability, so they are generally used to manufacture parts such as shafts, valves, and springs.
(3) 1Crl8Ni9, 1Cr18Ni9Ti
These materials have both certain toughness and good weldability, and they exhibit excellent corrosion resistance to various foods and cleaning solutions encountered in dairy production. Therefore, they are widely used in the food production industry.
The meanings of the symbols listed above are as follows:
The first digit represents the percentage of carbon in the metal material, with 0 indicating less than 0.1%, 1 indicating 0.1% to 0.15%, and 2 indicating 0.16% to 0.24%.
The second digit is the element symbol Cr for chromium steel.
The third digit is the percentage of chromium content in the metal, such as 13 representing 12% to 14% and 18 representing 17% to 19%.
The fourth digit is the element symbol for nickel.
The fifth digit is the percentage of nickel content, such as 9 representing 9% to 11%.
The sixth digit is the element symbol for titanium.
The designations of stainless steel in some countries are shown in Table 1-1.
2. Applications of Stainless Steel
In dairy plants, stainless steel is used most widely. All surfaces that come into contact with raw materials, semi-products, and products should be made of stainless steel.
3. Maintenance and Use of Stainless Steel
Stainless steel is widely used in dairy plants, but it should be noted that stainless steel does not always maintain its "rust-proof" properties under all conditions and environments. If used and maintained improperly, stainless steel can also corrode.
(1) Methods of Corrosion of Stainless Steel Equipment
1. Local Corrosion
This type of corrosion primarily occurs at welding joints. During welding, the metal is heated and oxidized, leading to local corrosion due to the different electrochemical properties of different substances. Prolonged high-temperature welding can cause crevice corrosion around the welds. The weld material's corrosion resistance is lower than that of the base material, especially under the action of chlorides, where this material may be decomposed and corroded.
2. Pitting Corrosion
Pitting corrosion on the surface of stainless steel is generally caused by abrasion from hard metals or other hard objects. In welding, pitting corrosion is caused by slag or oxides during the welding process. Similarly, under these conditions, especially at high melting temperatures, chlorides have a greater destructive or corrosive effect. During pressure testing of pipes, if water containing chlorine is not completely drained, it can also cause pitting corrosion. In high-salt environments, stainless steel is also prone to pitting corrosion.
3. Stress Corrosion Cracking
Under severe corrosion conditions, stress corrosion cracking can occur during steel production, welding, and load-bearing operation. High-temperature chloride solutions are the most common and severe corrosion environments.
(2) Factors Affecting the Service Life of Stainless Steel
1. Stainless Steel Welding Process
When welding stainless steel, inert gases are used, and their passivation must be 100% complete. Otherwise, stainless steel will lose its "rust-proof" properties in the welded area.
2. Stainless Steel Pipe Installation Process
The correct installation of stainless steel pipes in dairy plants is very important. After cleaning, liquid should be avoided in the pipes.
3. Stainless Steel Pipe Usage Process
(1) Cleaning. Do not use hydrochloric acid without corrosion inhibitors for cleaning. If acid cleaning is necessary, dilute nitric acid or phosphoric acid is better.
(2) Sterilization. If the surface of stainless steel comes into contact with calcium chloride, sodium chloride, or hypochlorous acid and its salts, it must be thoroughly rinsed before heating for sterilization. When using any hypochlorite solution for sterilization on the surface of stainless steel, the pH should preferably be controlled above 9. Hypochlorites mixed with potassium permanganate will enhance the corrosion of stainless steel.
(3) Cooling. When using brine with sodium chloride or calcium chloride, it is not advisable to come into contact with stainless steel if the pH is below 8.5.
4. Reasons for Corrosion of Stainless Steel
Stainless steel equipment in dairy workshops may suffer corrosion due to the following reasons, affecting its service life.
(1) Using wire brushes or hard metal wire wool for scrubbing, causing damage to the stainless steel surface and reducing its smoothness.
(2) Solids in hard water depositing on the metal surface, forming scale, and then being scraped off mechanically, causing surface damage.
(3) Improper use of chemicals for descaling or cleaning, or using chemical disinfectants for sterilization.