From a corrosion standpoint, the environments likely to be encountered in the food industry that may cause premature equipment failure may be classified under main headings:
Noncorrosive Mildly corrosive Highly corrosive Service fluids Alkaline detergents Acidic detergents Sanitizing agents
In general terms, natural foodstuffs such as milk, cream, natural fruit juices, and whole egg do not cause corrosion
problems with 304 or 316 stainless steels. Prepared foodstuffs to which there is no added salt such as yogurt, beer, ice cream, wines, spirits, and coffee also fall within this classification. For general storage vessels, pipelines, pumps, fittings or valves, grade 304 is perfectly satisfactory. However, for plate heat exchangers that are highly creviced and therefore prone to crevice corrosion, grade 316 frequently is employed. This offers a higher degree of protection against some of the more acidic products such as lemon juice, which may contain small quantities of salt and also provides a higher level of integrity against corrosion by service liquids and sanitizing agents.
It is quite common to use sulfur dioxide or sodium bisulfite for the preservation of fruit juices and gelatin solutions and in such cases, storage vessels always should be constructed from 316. Although the sulfur dioxide is non-corrosive at ambient temperature in the liquid phase, as a gas contained in significant quantities within the head space in a storage tank it tends to dissolve in water droplets on the tank wall. In the presence of air, the sulfurous acid that forms is oxidized to sulfuric acid at a concentration high enough to cause corrosion of 304 but not of 316.
This category of foodstuffs covers products containing relatively low levels of salt and where pH values are below 7. Examples include glucose/fructose syrups and gelatin, the production of which may involve the use of hydrochloric acid. For storage vessels, pipelines, fittings, and pumps, grade 316 has established a good track record, and boiling pans in this grade of steel are perfect for long and satisfactory service. The corrosion hazards increase however in processing operations involving high temperatures and where the configuration of the equipment is such as to contain crevices, especially when the product contains dissolved oxygen. For example, multistage evaporators operating on glucose syrup will usually have the first stage, where temperatures may approach 100°C (212°F), constructed in a super stainless steel such as 904L. Subsequent effects where temperatures are lower and where the product has been deoxygenated may be fabricated in grade 316 stainless steel.
As previously indicated, it is common practice to use sulfur dioxide as a preservative in dilute gelatin solutions during storage prior to evaporation. In some cases, excess hydrogen peroxide will be added to neutralize the sulfur dioxide immediately before concentration. This can have a catastrophic effect on the 300 series stainless steels and indeed on even more highly alloyed metals such as 904L, because of the combined effect of the chlorides present with the excess hydrogen peroxide. Because of this, it is a more acceptable practice to make the peroxide addition after rather than before evaporation.
Gelatin for pharmaceutical end use is subject to UHT treatment to ensure sterility. This will involve heating the gelatin solutions to 135°C (285°F) and holding at that temperature for a short period of time. Plate heat exchangers are used extensively for this duty. Although plates made from 316 stainless steel give a reasonable life of typically 2-3 years, a corrosion resistant alloy with an enhanced-level molybdenum is to be preferred.
The list of foodstuffs falling in this category is almost endless—gravies, ketchups, pickles, salad dressings, butter, and margarine—in fact, anything to which salt has been added at the 1-3% level or even higher. Also within this category must be included cheese salting brine and other brines used in the preservation of foodstuffs that undergo pasteurization to minimize bacterial growth on food residues remaining in the brine. Although these brines are usually too strong to support the growth of common organisms, salt resistant strains (halophiles) are the major problem.
Low-pH products containing acetic acid are particularly aggressive from a corrosion standpoint, but selection of materials for handling these products depends to a great extent on the duty involved.
When trying to define the corrosion risk to a piece of equipment handling potential corrodents, several factors come into play. While temperature, oxygen content, chloride content, and pH are the obvious ones, less obvious and equally important is contact time. All three main forms of corrosion induced in stainless steels (crevice, pitting, and stress corrosion cracking) have an induction period before the onset of corrosion. This can vary from a few hours to several months depending on the other operative factors. In a hypothetical situation where stainless steel is exposed to a potentially corrosive environment, removal of the steel and removal of the corrodents will stop the induction and the status quo is established. On repeating the exposure, the induction period is the same. In other words, the individual periods the steel spends in contact with the corrodent are not cumulative and each period must be taken in isolation.
When the contact period is short, temperatures are low and a rigorous cleaning regime is implemented at the end of each processing period, 316 stainless steel will give excellent service. However, where temperatures are high and contact periods are long, the corrosion process may be initiated. This is especially so in crevices such as the interplate contact points on a plate heat exchanger where, albeit at a microscopic level, corrodents and corrosion products are trapped in pits or cracks. Geometric factors may prevent the complete removal of this debris during cleaning and under such circumstances, the corrosion process will be ongoing.
Because of the perishable nature of foodstuffs, storage is rarely for prolonged periods or at high temperatures, and regular, thorough cleaning tends to be the norm. The one exception to this is buffer storage vessels for holding "self-preserving" ketchup and sauces. For such duties, an alloy such as 904L, Avesta 254 SMO, or even Inconel 625 may be required.
While all the foregoing applies to general equipment, the one exception is plate heat exchangers. Their highly creviced configuration and the high temperatures employed render them particularly susceptible. Plates made from grade 316 have a poor track record on these types of duties. Even the more highly alloyed materials do not offer complete immunity. The only reasonably priced material that is finding increased usage in certain areas of food processing is titanium.
The fact that butter and margarine have been included in this group of corrodents requires comment. Both these foodstuffs are emulsions containing typically 16% water and 2% salt. A fact not often appreciated is that the salt is dissolved in the water phase, being insoluble in the oil. From a corrosion standpoint, therefore, the margarine or butter may be regarded as a suspension of 12% salt solution and as such is very corrosive to 316 stainless steel at the higher processing temperatures. The only mitigating feature that partly offsets their corrosivity is the fact that the aqueous salty phase is dispersed in an oil rather than the reverse and that the oil does tend to preferentially wet the steel surface and provide some degree of protection. However, the pasteurizing heat exchanger in margarine rework systems invariably has titanium plates as the life of 316 stainless steel is limited and has been known to be as little as 6 weeks.
Steam. Because it is a vapor and free from dissolved salts, steam is not corrosive to stainless steels. Although sometimes contaminated with traces of rust from carbon steel steam lines, in the author's experience no case of corrosion due to industrial boiler steam ever has been encountered.
Water. The quality and dissolved solids content of water supplies varies tremendously, with the aggressive ionic species, chloride ions, being present at levels varying between zero, as found in the lakeland area of England, to several hundred parts per million, as encountered in coastal regions of Holland. It is also normal practice to chlorinate potable water supplies to kill pathogenic bacteria with the amount added dependent on other factors such as the amount of organic matter present. However, most water supply authorities aim to provide water with a residual chlorine content of 0.2 ppm at the point of use. Well waters also vary in composition depending on the geographical location, especially in coastal regions where the chloride content can fluctuate with the rise and fall of the tide.
What constitutes a "good" water? From a general user viewpoint, the important factor is hardness, either temporary hardness caused by calcium and magnesium bicarbonates that can be removed by boiling, or permanent hardness caused by calcium sulfate that can be removed by chemical treatment. While hardness is a factor, chloride content and pH probably are the most important from a corrosion standpoint.
What can be classed as a noncorrosive water supply from a stainless-steel equipment user? Unfortunately, there are no hard-and-fast rules that will determine whether corrosion of equipment will occur. As has been repeatedly stated throughout this article, so many factors come into play. The type of equipment, temperatures, contact times, etc all play a role in the overall corrosion process. Again, as has been stated before, the most critical items of equipment are those with inherent crevices— evaporators and plate heat exchangers among others. Defining conditions of use for this type of equipment will be the regulatory factor. Even then, it is virtually impossible to define the composition of a "safe" water, but as a general guideline, water with less than 100 ppm chloride is unlikely to initiate crevice corrosion of type 316 stainless steel while a maximum level of 50 ppm should be used with type 304.
Cooling tower water systems are frequently overlooked as a potential source of corrodents. It must be appreciated that a cooling tower is an evaporator, and although the supply of makeup water may contain only 25 ppm chloride, over a period of operation this can increase by a factor of 10 unless there is a routine bleed on the pond.
Water scale deposits formed on heat-transfer surfaces should always be removed as part of the routine maintenance schedule. Water scale deposits can accumulate chloride and other soluble salts that tend to concentrate, producing higher levels in contact with the metal than indicated by the water composition. Furthermore, water scale formed on a stainless-steel surface provides an ideal base for the onset of crevice corrosion.
As stated previously, potable water supplies usually have a residual free chlorine content of 0.2 ppm. Where installations have their own private wells, chlorination is undertaken on site. In general terms, the levels employed by the local water authorities should be followed and over-chlorination avoided. Levels in excess of 2 ppm could initiate crevice corrosion.
Cooling Brines. Depending on the industry, these can be anything from glycol solutions, sodium nitrate/carbonate, or calcium chloride. It is the latter which are used as a 25% solution that can give rise to corrosion of stainless steel unless maintained in the ideal condition, especially when employed in the final chilling section of plate heat exchangers for milk and beer processing. However, by observing certain precautions, damage can be avoided.
The corrosion of stainless steels by brine can best be represented as shown in Figure 22. It will be noted that an exponential rise in corrosion rate with reducing pH occurs in the range pH 12-7. The diminution in number of pits occurring in the range pH 6-4 corresponds with the change in mode of attack, ie, from pitting to general corrosion. It will be seen that ideally the pH of the solution should be maintained in the region 14-11. However, calcium chloride brine undergoes decomposition at pH values higher than 10.6:
CaCl2 + 2NaOH Ca(OH)24 + 2NaCl
Scale deposition occurs and heat-transfer surfaces become fouled with calcium hydroxide scale. Furthermore, the scale that forms traps quantities of chloride salts that cannot be effectively removed and remain in contact with the equipment during shutdown periods. This is particularly important in equipment such as plate heat exchangers that are subjected to cleaning and possibly hot-water sterilization cycles at temperatures of 80°C (176°F) or higher.
The other aspect, nonaeration, is equally important. Air contains small quantities of carbon dioxide that form a slightly acidic solution when dissolved in water. This has
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