Stainless Steel sheets belonging to the category of Cr-Mn-Ni-N and manufactured by several indigenous manufacturers are finding application in utensils industry. These steel sheets were called 200 Series of sheets. These were cheaper to make as the costly Nickel in it was replaced by Manganese. It was argued that this kind of steel was inferior then the original 300 Series of steel.

Thus to determine the true facts, it was desired to carry out certain tests which would find the suitability of these indigenous manufactured grades for dissolution of its elements in foods and beverages.

Steel specimens of 2 different 200 series grade were compared with a known 300 series grade of steel.The tests made were mainly to find out the corrosion behavior, formability and processing of these 3 grades of Steel.

Tests and their Results -

Corrosion Under total immersion condition in 3% NaCl solution

Experiments were carried out at room temperature for 720 hours and at boiling temperatures for 100 hours.
At room temperature all the 3 specimens exhibited localized attack. Metal loss due to corrosion was negligible of the order of 0.01 mm per year. At boiling point no specific feature was found.

Corrosion under total immersion in organic acid mixture
( 0.5% acetic acid + 0.5% lactic acid)
Experiments were carried out at room temperature for 720 hours and at boiling temperatures for 100 hours.
Corrosion under total immersion in organic acid mixture ( 0.5% acetic acid + 0.5% lactic acid)
Corrosion under total immersion in organic acid mixture
(0.5% citric acid + 0.5% tartaric acid)
Experiments were carried out at room temperature for 720 hours and at boiling temperatures for 100 hours. Metal loss due to corrosion was negligible of the order of 0.01 mm per year. No specific corrosion feature was observed.
Corrosion under total immersion in, organic acid mixture 1 +3% NaCI
Experiments were carried out at room temperature for 816 hours. This surface was bright although some rust spots were observed. Corrosion rate continued to be a fraction of 0.01 mm per year.
Corrosion under total immersion in organic acid mixture 2 + 3% NaCI
Experiments were carried out at room temperature for 816 hours. The surface was bright although some rust spots were observed. Corrosion rate remained a fraction of 0.01mm per year.
Corrosion under partial immersion in organic acid mixture 1.
Experiments were carried out at room temperature for 720 hours and at boiling point for 100 hours. No rust spots were observed. Water-line stain was observed at boiling point.
Corrosion under partial immersion in organic acid mixture 2.
Experiments were carried out at room temperature for 720 hours and at boiling point for 100 hours. No rust spots were observed. Water-line stain was observed at boiling point.
Stress Corrosion cracking lest
Tests were conducted for 30 days at room temperature and for 100 hours at boiling point in all the three media. No cracks were observed visually. No stress corrosion crack could be observed even at a magnification of 500X.
High Temperature oxidation
Experiments were done at 500 c and 800 c. Weight gain was negligible. Thick opaque film with flame lining was observed at 800c. Dark blue color film with flame lining was observed at 500 c. This shows that under direct heating in dry condition appearance of the three steels is affected. When the temperature of the heated vessel with water inside was measured, only a temperature of 200 c was recorded on the outer surface. Slight bluish color was observed under these conditions. No color change was observed on the outer surface after boiling water for 6 hours.


Top


Dissolution Behavior of the
three specimens in the three media

1) 3% NaCI
Experiments were conducted for 720 hours at room temperature and 100 hours at boiling point.
Cr and Cu were present in part per billion levels only.
Fe was present to the extent of less than 1 p.p.m.
Mn was present to the extent of less than 3 p.p.m. At room.
Temperature but it increased to 2 p.p.m. at boiling point.
Similar results were observed in both total and partial immersion conditions
2) Organic Acid Mixture I
Metal dissolution was less than I p.p.m.
Similar results were observed both total and partial immersion
3) Organic Acid Mixture II

Metal dissolution was of the order of I p.p.m. and less.
Similar results were observed in both total and partial immersion.

Discussions from the Results

  1. A corrosion rate of the order of 10' mm per year in NaCI solution has no significance when it comes to contact with the same media for several minutes or several hours or even several days. Hence the data collected must be looked at from a more practical point of view.

  2. There is no risk of stress corrosion cracking of these steels when cold worked but not annealed during fabrication into utensils.

  3. The study on the dissolution of the elements present in the steels in NaCI solution and organic acid mixture shows that at room temperature pick up of Cr, Mn, Ni is negligible irrespective of the exposure period increasing from 100 hours to 1000 hours.

  4. The electrochemical measurements are most illuminating. There is clear evidence that incipient dissolution of Iron during the first use of utensil made of stainless steel should not be mistaken for tendency to corrosion. With cleaning and drying frequently the steel would acquire a naturally former protective oxide film which will prevent further The corrosion rate obtained in electrochemical measurement also indicates that corrosion is insignificant


Top

Conclusions & Recommendations

  • The most aggressive conditions that may occur in handling food & beverages have been visualized in the experimental program

  • Experiments show that all the three specimens studied withstand any attack by the organic acids likely to be present in food stuffs

Thus it was proven that Stainless steel specimens containing
Fe-Ni-Cr-Mn-Cu-N2 (broadly known as 200 Series) have performed well in comparison with 304 series (Standard) of Stainless Steel


Top