The chemical composition of stainless steel

by | Jul 1, 2020 | Practical in the cellar, Winetech Technical

Stainless steel is the most popular material for the production of equipment in wine cellars and the beverage industry. It is especially the ease of cleaning and its inert properties that are responsible for its popularity. Its chemical composition and structure can however differ, depending on the aim of its use.

The two grades of stainless steel that are most used in the wine industry are 304 and 316 stainless steel. Both can be manufactured in standard or low carbon content. Their composition differs, which imply the efficiency of their use in a wine cellar or for the storing of high concentration chemicals. Grades 304 and 316 stainless steel are part of the austenite family, while other alloys and chemicals form the ferritic, duplex and martensitic families of metals. Wine does not corrode 304 stainless steel in spite of its high acid concentration and it is consequently the most popular grade of stainless steel in the wine industry, due to its formability and weldability.

Three types of grade 304 stainless steel exist, namely 304, 304L and 304H, which differ in their carbon content. 304L is a low carbon type, which may not contain more than 0,03 carbon density. The metal composition of the different stainless steel grades are indicated in Table 1.

As already mentioned, wine cannot corrode grade 304 stainless steel. If more than 700 mg/L sulphur dioxide however occurs the 304 grade stainless steel will be corroded.

Grade 304 is cheaper than grade 316, because it contains less nickel. Seeing that it does not contain molybdenum, it is more exposed to corrosion, which can cause pitting and crevices in the metal if it is exposed to chloride. Grade 304 can resist water with a chloride concentration of 200 ppm, while grade 316 can resist water with a chloride concentration as high as 1 000 ppm. Seeing that grade 316 contains molybdenum, it is less exposed to this corrosion. The difference in the composition of stainless steel can lead to a price difference of 25% in the cost of material. Apart from the mentioned differences, the two grades of stainless steel have the same formability and welding properties. Due to the sulphur dioxide resistance of grade 316, it is recommended if the total sulphur dioxide concentration of wine is higher than 75 mg/L. Seeing that high sulphur dioxide concentrations may occur in the ullage space of tanks which are not full, the domes of such tanks can sometimes be made from grade 316. It is unnecessary in most cases, because the ullage space is usually filled with inert gas.

Although both grades are acceptable for wine cellar tanks, unless high acid concentrations are stored, 304 is usually preferred as a result of its characteristics and price advantage. If larger tanks are preferred, it is recommended to use stronger duplex types.

The cleaning and maintenance of stainless steel tanks are essential to extend their lifetime. It is important that untreated water is not left in stainless steel tanks. Depending on its salt concentration, it can especially during the summer months evaporate and concentrate the salt concentration in the remaining water. This creates an environment that will cause the rusting of stainless steel. It is consequently recommended that a weak caustic soda solution is used for cleaning, followed by a water rinse, before it is dried. Stainless steel must never be left wet and must also be left open for ventilation.

Apart from the cleaning of stainless steel tanks, it must always be remembered to leave the vents open before transferring wine from it to prevent the imploding of the tank (Szentpeteri, 2018).

Stainless steel is associated with cellar equipment.

Reference

Szentpeteri, C., 2018. Steel yourself for double duty tanks. Australian and New Zealand Grapegrower and Winemaker, September 2018: 60 – 62.

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