A

Add more molybdenum for pitting resistance

Nickel, chromium, iron (molybdenum, copper, niobium) alloys

Add nickel, molybdenum, copper, niobium t-- for corrosion —^ 317 resistance in reducing environments

Increased chromium lower nickel for special properties

Add copper, titanium, aluminum, lower nickel for — precipitation hardening (corrosion resistance reduced)

Add manganese and nitrogen, lower nickel for higher strength (corrosion resistance reduced

Duplex stainless steels (329)

Precipitation hardening stainless steels

Austenitic iron, nickel, manganese nitrogen, stainless steels

Figure 2. Outline of some compositional modifications of 18/8 austenitic stainless steel to produce special properties. Source: Ref. 2.

Still further increases in the three aforementioned alloying elements result in the nickel alloys. (The classification of an alloy is generally under the heading of the major constituent.)

There are a large number of these alloys but those of primary interest to the food industry are shown in Table 2 together with their composition. In general terms, it will be noted that the increase in nickel content is accompanied by an increase in chromium and molybdenum. As stated previously, this element is particularly effective in promoting corrosion resistance.

Just like insurance, you get only what you pay for, and generally speaking, the higher the corrosion resistance, the more expensive the material. In fact, the differential between type 304 stainless steel and a high-nickel alloy may be as much as 20 times, depending on the market prices for the various alloying elements that fluctuate widely with the supply and demand position.

Aluminum

High-purity grades of aluminum (± 99.5%) and its alloys still are preferred for some food and pharmaceutical applications because of the reasonable corrosion resistance of the metal. This resistance is attributable to the easy and rapid formation of a thin, continuous, adherent oxide film on exposed surfaces. This oxide film, in turn, exhibits a good corrosion resistance to many foodstuffs, and it is reported that fats, oils, sugar, and some colloids have an inhibitory or sealing effect on these films (3).

As aluminum salts formed by corrosion are colorless, tasteless, and claimed to be nontoxic, the metal is easy to clean, inexpensive, and light and has a high thermal conductivity. It still is used quite extensively in certain areas of food manufacture and distribution. However, in recent years, the claim of nontoxicity is being questioned as a high dietary incidence has been implicated in Alzheimer's disease (senile dementia) with compounds of aluminum (aluminosilicates) being found in the brain tissue of sufferers (4). However, the case is far from proven, and it is not clear if the increased levels of aluminosilicates are due to a high intake of aluminum per se or other factors such as a dietary deficiency of calcium.

For many years, aluminum was used extensively for containment vessels in the diary and brewing industry, and it was Richard Seligman who founded the then Alu minium Plant and Vessel Company (now APV pic) to exploit the technique of welding this material for the fabrication of fermenting vessels in the brewing industry (5). Many of these original vessels are still in use in some of the smaller, privately owned breweries in the United Kingdom.

Although large fermenting vessels and storage tanks now tend to be fabricated from stainless steels, there is still widespread use of aluminum for beer kegs, beer cans, and a miscellany of small-scale equipment where the resistance of aluminum is such that it imparts no change or modification of flavor, even after prolonged storage.

While still used for holding vessels and some equipment when processing cider, wines, and perry, prolonged contact is inadvisable because of the acidity of the sulfites employed as preservatives for these products—inadvisable, that is, unless the surface of the metal has been modified by anodizing or has been protected with a lacquer.

In the manufacture of preserves, aluminum is still employed for boiling pans, the presence of sugar appearing to inhibit any corrosion. In the field of apiculture, it has even been used for making prefabricated honeycombs, which the bees readily accepted.

Extensive use is made of aluminum and the alloys in the baking industry for baking tins, kneading troughs, handling equipment, etc.

In other areas of food manufacture and preparation, the use of aluminum extends virtually over the whole field of activity—butter, margarine, table oils, and edible fats, meat and meat products, fish and shellfish, certain sorts of vinegar, mustards, spices; the list is almost endless.

No mention has so far been made of the application of this metal in the dairy industry, and indeed it still has limited application mostly in the field of packaging, eg, bottle caps, wrapping for cheese, butter, carton caps for yogurt, cream.

It will be appreciated that the uses of aluminum in the food industry so far mentioned have tended to be for equipment used in batch operation, hand utensils, and packaging. There are probably three major factors that have mitigated against its more widespread use, not only in the dairy industry but in brewing and many other branches of food processing.

1. Modern, highly automated plants operating on a continuous or semicontinuous basis employ a wide va

Table 2. Composition of Some of the More Commonly Used Wrought Super Stainless Steels and Nickel Alloys

Composition (%)

Table 2. Composition of Some of the More Commonly Used Wrought Super Stainless Steels and Nickel Alloys

Composition (%)

Alloy

UNS no.

Carbon

Silicon

Manganese

Chromium

Nickel

Molybdenum

Others

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