Corrosion And Food Processing

MATERIALS OF CONSTRUCTION The "Stainless Steels"

Many people refer to this range of alloys as austenitic stainless steels, 18/8s, 18/10s, etc, without a full appreciation of what is meant by the terminology. It is worth devoting a few paragraphs to explain the basic metallurgy of stainless steels.

It was in 1913 that Harry Brearley discovered that the addition of 11% chromium to carbon steel would impart a good level of corrosion and oxidation resistance, and by 1914 these corrosion resisting steels had become commercially available. It was Brearley who pioneered the first commercial use of these steels for cutlery, and it was also he who coined the name "stainless steels." For metallurgical reasons, which are outside the scope of this article, these were known as ferritic steels because of their crys-tallographic structure. Unfortunately, they lacked the ductility to undergo extensive fabrication and furthermore, they could not be welded. Numerous workers tried to overcome these deficiencies by the addition of other alloying elements and to produce a material where the ferrite was transformed to austenite (another metallurgical phase) that was stable at room temperature. Soft stainless steels that were ductile both before and after welding were developed in Sheffield, England (then the heart of the British steel industry) exploiting scientific work undertaken in Germany. This new group of steels was based on an 18% chromium steel to which nickel was added as a second alloying element. These were termed the austenitic stainless steels. The general relationship between chromium and nickel necessary to maintain a fully austenitic structure is shown diagrammatically in Figure 1. It will be seen that the optimum combination is 18% chromium, 8% nickel— hence the terminology 18/8s.

Probably the next major advance in the development of stainless steels was the discovery that relatively small additions of molybdenum had a pronounced effect on the corrosion resistance, greatly enhancing the ability to withstand the effects of mineral acids and other corrodents such as chloride solutions. Needless to say, from these early developments, there has been tremendous growth in production facilities and the number of grades of stainless steel available. Table 1 lists some of the more commonly available grades, while Figure 2 illustrates how the basic 18/8 composition is modified to enhance specific physical or chemical properties.

In spite of the plethora of stainless steels available, grades 304 and 316 have, and continue to be, the workhorses for fabrication of dairy and food processing equipment.

Although 316 stainless steel offers excellent resistance to a wide range of chemical and nonchemical environments, it does not offer immunity to all. In the case of the food industry, these are notably anything containing salt, especially low-pH products. There was, therefore, a demand by industry to develop more corrosion-resistant materials and these are finding increasing use in the food industry for certain specific processing operations.

Super Stainless Steels and Nickel Alloys

The super stainless steels are a group of alloys that have enhanced levels of chromium, nickel, and molybdenum, compared to the conventional 18/8s. The major constituent is still iron; hence the classification under the "steel" title.

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