Stainless steel

THE DIFFERENT TYPES OF STAINLESS STEELS

Stainless steel is mainly an iron-chromium alloy with more than 11% chromium. Here are the different types of steels: 

1. Austenitic steels

Austenitic steels are made up of 18 to 20% chromium and 8 to 10% nickel. These are non-magnetic and therefore show no response to a magnet. The most common ones are the 18/8 or 18/10 grades (AISI 304). The nickel content makes this type of steel the only one that can guarantee full corrosion resistance. Austenitic steel is used for the production of upmarket stainless steel cutlery. 

2. Martensitic steels

Martensitic steels are made up of 10 to 18% chromium and can be tempered (i.e. hardened). It is the most common used in knife production (AISI 420). This type of steel is essential to produce good quality knives for which the blades have to be hardened in order to guarantee an effective, long-lasting cutting edge, which does not attack earthenware or china.

However, it is the least corrosion resistant. When the dishwashers appeared on the market, it was therefore recommended not to put knives in dishwashers.

3.  Ferritic steels

Ferritic steels contain 16 to 18% chromium but can not be tempered. This type of steel is a bit more corrosion resistant than martensitic steel and can be cold-worked. AISI 430 is the most common used in the production of low-priced household items

We can recognise these two types of steel (Martensitic and ferritic steels) thanks to their magnetic response. The experience of several manufacturers tends to show that these two types of steel are dishwasher-proof but weaken when some adverse elements are gathered (see chapter 2: Oxidation).

In order to reduce the risks of corrosion, Eternum is using a high chromium content steel (17 to 18%) for low-priced cutlery (ferritic steel – no nickel). This type of steel is more expensive and we use it for most of the Eternum range. For Eternum Signature knives, we would use chromium-molybdenum steel, even more expensive.

All the steels that we buy are carefully checked. A Certificate of Analysis guarantees that the quality meets the required standards.

In addition, we are using a spectrometer in order to check the steel composition and make sure that the steel of our products meets the international standards. 

In practical terms, before each analysis the machine is standardized thanks to a benchmark which content is perfectly known and certified. The results of the analysis and the differences with the benchmark allow us to confirm whether the steel used meets the current standards. 

OXIDATION

First of all, it is important to point out that corrosion does not depend on the stainless steel quality. In order to fully understand the factors that might generate corrosion, it’s necessary to know what makes steel stainless.

As we know, stainless steel is an alloy whose main component is iron. It is made stainless thanks to a self-protecting mechanism that occurs with oxygen or in an oxidizing environment (surrounding air).

Unlike common steels, where rust that develops on the surface can penetrate into the entire mass, stainless steels can create a thin, continuous and tight chromium oxide layer preventing oxidation from penetrating the steel. Sufficient chromium content makes this phenomenon, “THE PASSIVATION”, possible.

To sum up, full corrosion resistance can only be achieved if a very thin protective passive layer can develop itself on a clean surface by free oxidation. 

It is therefore imperative to avoid or remove all residues from the cutlery. There are 4 types of residues:

1. Food residues due to defective cleaning:

These can usually be avoided by rinsing the cutlery before placing them in the dishwasher in order to prevent food residues from sticking to the cutlery.

2. Food residue from new deposit:

The cutlery is properly washed during the cycle but a poor rinsing does not remove all the dirt, which is re-deposited on the dishes. In order to avoid this type of deposit it’s recommended to carefully follow the washing conditions in the instructions for use.

3. Scale deposits:

Scale deposits are quite common when the water is too hard. These can be avoided by using a water-softening installation.

4. Deposits due to a too high salt content in water:

These can be avoided by installing desalting equipment. 

Another major source of problems is the “depassivation” of stainless steel. As the steel is stainless thanks to a very think protective passive chromium oxide layer it is very important not to put it together with elements that could activate it while changing its chemical composition: chloride (salt), iron (mayonnaise lids), silver and aluminium (aluminium deposits will give a matt effect to the steel). 

In addition, ferritic (no nickel) and martensitic (knives) steels are likely to be attacked by:

  • A contact with cooking acids (vinegar, lemon, tomato sauce, mayonnaise, etc.);
  • An inappropriate rinsing product quantity (most often acid-citric based!)
  • An inappropriate washing product quantity

Since the first appearance of dishwashers, the load has been increasing while the household size has decreased. As a consequence, there is a longer period between the cutlery being placed in the dishwasher and the washing. Therefore, the food residues dry over several days and require more aggressive product to be removed. As the passivation that makes steel stainless is only possible when the steel is in contact with the surrounding air, the stains remaining on the cutlery prevent the steel from protecting itself. Similarly, we are all familiar with the advertisings for miracle products to wash more with fewer products. However, in order to get a better result by using fewer products, the product itself needs to be more aggressive. In addition, there is a tendency to use the exact same quantity, no matter the product. This is why the consumer will usually use much more product as what is shown in the advertising. Too much product would visibly attack certain steels and weaken glassware and tableware (earthenware and china).

  • An inappropriate rinse

As washing and rinsing products are corrosive, it is very important to rinse properly. It’s recommended to wash no nickel cutlery and knives in an upright position so that water can flow. Indeed, water “blocked” in between cutlery may attack steel. 

A few tips…

Here are a few tips to avoid any unpleasant surprises:

  • Wash the cutlery as promptly as possible after use. If this is not possible, rinse the cutlery to prevent prolonged contact with cooking acids.
  • Make sure your dishwasher is working properly. If necessary, have it tuned by your technician taking your tap water into account (the standard settings do not necessarily correspond to the water you use).
  • Gradually reduce the amount of washing and rinsing products until you reach the minimum satisfactory quantity.
  • Avoid putting the cutlery in contact with iron or silver articles.
  • Remove your cutlery from the dishwasher as soon as it is dry (usually about 15 minutes after the end of the cycle). Leaving the cutlery in the damp atmosphere is the worst thing to do.
  • Never use too aggressive products (caustic soda, for example), even for articles in 18/8 or 18/10 steel.
  • Rinse and dry the cutlery if powder or food residues remain (especially on the blades of knives and on low-cost cutlery). There are different techniques to remove the spots:

- rub them off by using a stainless steel cleaner, available in hardware shops.

- rub the spot with a cotton pad soaked in vinegar, rinse and dry thoroughly;

- cut an onion in half, put some caster sugar on it and rub the spot. Then rinse and dry.

                       

CONCLUSIONS

We do hope that you have now become more familiar with stainless steel and the best way to use it. A leaflet version of this report is also available upon request.  Do not hesitate to provide it to your customers. It would help to easily prevent problems that might happen with low-cost cutlery and knives.

We devote all our experience as silversmiths since 1924 and use all the best materials to offer you a product with the best possible value for money. By following the above recommendations, the user will preserve all the intrinsic qualities of his cutlery.

N.B: We can not be held responsible for any problems due to a bad usage.