In last month’s Cape Business News, I talked about the supplier’s importance as a partner in running a factory and minimising breakdowns. I mentioned the need for technical advice as part of the package which suppliers should offer and which NDE Stainless Steel provides as a free and much-valued service to customers
Technical advice is an important aspect in ensuring the success of any process, in any industry.
In the Food and Beverage, and Dairy Manufacturing industries there are many processes where corrosion is a factor, and the use of the correct stainless steel grade is crucial in providing solutions that are both corrosion resistant and hygienic.
As an example, I have highlighted fruit handling and washing because stainless steel is a commonly used material in the washing of fruit here in the Western Cape and elsewhere.
Fruit Handling and Washing
The most common corrosion problem with stainless steel in water-related applications for fruit washing is a corrosion problem known as pitting.
Pitting is one of several types of corrosion which may occur in stainless steel under certain circumstance – and which, in many cases, can be prevented or minimised, mainly through choice of the correct grade of stainless steel.
Pitting is caused by chlorides in the water. Obviously the higher the chloride content of the water, the greater the likelihood of pitting.
The other factor that can cause pitting is lack of oxygen in contact with the stainless steel.
Stainless steels become corrosion resisting because of the formation of a very thin (5 nanometers) chrome oxide (a ceramic) on the surface.
This oxide can be removed through abrasion (tumbling of fruit) or through chloride attack.
Once this oxide is removed the stainless steel needs to regenerate the oxide film which it can only do in the presence of oxygen.
The most common barrier to the re-forming of the chrome oxide in the water industry is stagnant water.
Selection of stainless steel grades
There are over 500 grades and finishes of stainless steel, with a core group of well-known and frequently used grades.
Grade 304L can be used where the water used for washing contains less than 200 ppm (parts per million). Most municipal water runs between 80 and 150ppm.
304L can safely be used at these chloride levels provided that the water is flowing and not allowed to stagnate for periods longer than a day.
It is recommended with stainless steel that the fruit washing equipment is designed in such a way that it can be totally drained when not in use.
If either the chloride content is higher than this or there is stagnation of water, then grade LDX2101 or 316L should be used.
When chlorides in water are higher than 600ppm (as in sea or borehole water), grade 2205 duplex stainless steel steel is suitable.
