APV founder: Dr. Richard Seligman APV has evolved and grown over the years to better meet the changing needs of our customers and their industries. The first commercially successful plate-and-frame heat exchanger was introduced in 1923 by the Aluminum Plant and Vessel Company Ltd., which became known as APV. The first Paraflow Plate Heat Exchanger, constructed of cast gunmetal plates and enclosed within a crude frame, set the standard for today’s computer-designed thin metal plate. Table 1: Composition of some of the more commonly used austenitic stainless steels. Unless indicated otherwise, all values are maximum. Fig. 2: Outline of some compositional modifications of 18/8 austenitic stainless steel to produce special properties (Ref. 2). The super stainless steels are a group of alloys which 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. 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 compositions. In general terms, it will be noted that the increase in nickel content is accompanied by an increase in chromium and molybdenum. As previously stated, this element is particularly effective i promoting corrosion resistance. Just like insurance, you only get what you pay for — and generally speaking, the higher For any processing operation, there will be a range of materials which will offer a corrosion resistance which is adequate (or more than adequate) for a particular job. When considering corrosion resistance, the operational environment is the obvious one. But, the other point must be whether the material will also offer corrosion resistance to the chemicals used for cleaning and sanitizing. s Cocact Another somewhat unique example of galvanic corrosion is related to a weld repair on < 304 stainless steel storage vessel. Welding consumables containing molybdenum had been employed to effect the repair. Although it is unusual for the steels to be sufficient to initiate galvanic corrosion, the environmental factors in this particular case were obviously such that corrosion was initiated (Figure 5). As stated, this is somewhat unique and it is not uncommon for 316 welding consumables to be used for welding 304 stainless steel with no adverse effects. As a practice, however, it is to be deprecated and the correct welding consumables should always be employed. Not all galvanic corrosion is bad; galvanic corrosion is used extensively to protect metal and structures by the use of a sacrificial metal coating. A classic example is the galvanizing of sheet steel and fittings, the zinc coating being applied not so much because it doesn’t corrode, but because it does. When the galvanizing film is damaged, the zinc galvanically protects the exposed steel and inhibits rusting. Similarly, sacrificial anodes are fitted to domestic hot water storage tanks to protect the tank. J. 7: Crevice corrosion at the interplate contact points of a heat exchanger plate ‘ig. 8: Photomicrograph of a section through a site of crevice corrosion. Note the deep undercutting which is typical of chloride-induced attack on stainless steel. Fig. 10: Elongated pitting attack on a 316 stainless steel heat exchanger plate. Table 4: The effect of alloying on pitting resistance of stainless steel alloys. Fig. 15: Photomicrograph of a weld deposit on a ferritic stainless steel. Compare the size of grains in the weld with those in the parent material on the left. Fig. 22: The effect of pH and temperature on the pitting of stainless steel by brine. ‘ig. 23: Catastrophic failure of the gasket groove of a heat exchanger plate by stress corrosion cracking caused by the use of a polychloroprene-based adhesive. Table 7: Suggested limits of concentration and temperature for peracetic acid in contact with rubbers.
