Maintaining Stainless Steel

Many believe that stainless steel is fully rust-proof, so it does not corrode easily. Stainless steel is an alloy that includes chromium and other elements, which provide some resistance against corrosion.

Image Credit: Shutterstock/AnaMarques

The base material is almost as active as the common carbon steel. An extremely thin passive chromium oxide layer on the surface of stainless steel contributes to the noble nature of this alloy. However, when this layer is absent, the material rusts quickly.

Proper care should be taken of this chromium oxide layer to ensure the durability of stainless steel. To put it simply, stainless steel is a low maintenance alloy.

A component based on stainless steel can be compared to a fresh apple that lasts for a long time because of its thin peel, yet no substances can enter or escape until an insect eats its way through the peel, initiating the rotting process. When the apple is cut in half, its flesh will rapidly oxidize.

The ‘skin’ on stainless steel is relatively thinner than the apple peel and still offers complete protection. As a result, no foreign substances can enter, and no metal ions can escape. Conversely, when the skin is subjected to a large amount of chemical load, it will decompose, causing corrosion.

The major difference is that stainless steel can repair its oxide layer on its own, unlike the apple. This is especially true when the layer is mechanically damaged and the layer of chromium oxide vanishes in local areas.

Due to atmospheric oxygen, material passivation will occur in those areas via the formation of a new chromium oxide layer. This mechanism is called the ‘self healing effect’, though it can be severely interrupted when chlorides are present.

The Solution

Stainless steel products and fabrications should be clean when they enter service and should be kept clean while in use, weld areas specifically should be free from contamination and discolourization. In addition, any mild contamination of steel that occur during the fabrication process should be fully removed.

This article describes products that are shown to be the best solution for cleaning and protecting stainless steel, as well as for preserving its attractive appearance. It also provides a complete technical explanation of the situation.

Dirt Deposits

Due to oxygen, stainless steel should be able to breathe as it exists. While oxygen helps to sustain the width of the oxide layer, it is a comparatively large molecule and should remain in contact with the surface. This does not pose any issue when the surface is clean, but places that are not exposed to rain are prone to under deposit corrosion.

Figure 1 shows a revolving door that was installed four years ago. It has a considerable amount of stainless steel and is located close to sea water.

Initially, it appears that there is nothing wrong with the door, but a closer look at the arch above the door shows a selection rusty spots or tea stains. As this revolving door is located close to the coast, the cause of destruction comes from a combination of aerosols and dirt deposits.

Figure 1. A revolving door with all kinds of rusty spots

Aerosols are tiny droplets of sea water that evaporate when airborne, boosting their concentrations of chlorides and salts. They accumulate on the surface of stainless steel, while the chlorides push their way through the dirt deposits in the pores to attack the material.

If this is not resolved, the corrosion will gradually spread further. In Figure 1, it can be seen that the stainless steel under the porch has experienced much more corrosion compared to the section exposed to rain.

This is because most of the salt residues are washed away by rain water, so the material suffers a lower degree of corrosion. However, these tea stains would not have developed if the stainless steel arch had been cleaned frequently. The cause is the dirt deposits in the pores.

As described earlier, the stainless steel surface contains a substantial amount of dirt, which can be viewed through a microscopic image. A magnification of 1500x reveals a considerable amount of dirt in the pores on the surface of the stainless steel and this should not be regarded as an exception. This is the rule rather than an exception.

Figure 2 displays a pictorial representation of a surface of stainless steel that has been significantly magnified. All kinds of dirt deposits are accumulated in the pores, through which chlorides will move easily.

Figure 2. A diagrammatic representation of a stainless steel surface

This will be relatively easier for small chlorine ions than for larger oxygen molecules, and that is where the problem lies. Similar to bromine, fluorine, and iodine, chlorine is a member of the halogen family, which are known as salt formers.

As a result, chlorine ions tend to merge with metals to develop metal chlorides and this is the case when oxygen is not allowed to reach the oxide layer to keep it in excellent condition. Subsequently, this layer will decompose, leading to under-deposit corrosion.

As soon as this surface is cleaned down to the pores, oxygen will be able to perform its job to ensure passivity.

This form of corrosion is shown in Figure 3, where a switchboard cabinet made from AISI316 steel is located in the open air close to the sea water.

It is believed that a stainless steel which is resistant to seawater should be selected in this situation. It is predominantly aerosols that attack this surface beneath the dirt deposits. This form of corrosion would not have developed if this surface had been cleaned frequently.

As the entrance gate in question is enclosed, rain water cannot easily reach the object to clean it more meticulously. It was mistakenly believed that stainless steel is a maintenance-free product.

Here, the corrosion can still be removed and is no reason to substitute this part. However, additional protection would have to be applied following cleaning, as small spots have formed that may rapidly lead to new corrosion.

Figure 3. A switchboard cabinet made from AISI316 steel

Surface Tension

All liquids have a certain surface tension, which can be distinctly viewed with mercury drops that form globules on a glass sheet. Droplets formed from water appear more like toadstools.

Unlike water, mercury will not wet the sheet of glass. This is because water has a low surface tension, while liquid mercury has a high surface tension. When soap is added to the water, the surface tension will be reduced further.

Tensides are referred to those substances that break or lower the surface tension. These can either be anionic or ionic. If the surface tension is lower, the agent will penetrate deeper into the pores. This is illustrated in Figure 4.

Figure 4. A. The surface tension is too high to reach the root of the pore. B. Innosoft B570 contains ionic and anionic tensides and that will break the surface tension in order to moisten and clean the bottom of the pores. Less risk of under deposit corrosion.

The top picture (Figure 4A) shows a cleaning agent with a moderately high surface tension and the bottom picture (Figure 4B) shows an optimum surface tension. The presence of a cleaning or detergent agent can do its job dissolving the dirt.

To put it simply, the pores are cleaned of dirt deposits as good as possible, allowing efficient control of under deposit corrosion. This is because oxygen can easily access the entire surface of the stainless steel. Once again, this highlights the importance of keeping the surface clean.

Cleaning and Passivation Kit for Stainless Steel

Aalco offers cleaning and passivation kit that removes contamination and corrosion from stainless steel surfaces (Figure 5).

Figure 5. Cleaning and passivation kit for stainless steel

Innosoft B570 is a unique deep cleaner that is capable of removing contamination like rust or oxygen from the surface of stainless steel in just one operation.

It is a white thick liquid based on surface active agents and organic acids. Innosoft B570 can be used for thorough cleaning of contaminated surfaces of rusty areas, including flash rust.

Innosoft B570 rust remover is not a pickling agent, and does not attack the metal. It is very effective on dirt and oxides, and non-aggressive on metals.

Although Innosoft B570 is a new product, it is already known for its effective removal of contaminations. It includes unique tensides, which allow the agent to deeply enter into the pores.

In addition, this agent includes a strong detergent that completely dissolves dirt, which helps to prevent the possibility of ‘under deposit corrosion’.

While Innosoft B570 is suitable for removing rusty corrosion products, it can also be used as a preventative measure to eliminate flash rust as much as possible. Flash rust can be further prevented if the liquid Innoprotect B580 is applied after using the Innosoft B570 product.

This liquid offers additional protection on a nanoscale, and can be compared to an atomic bonded layer that enables the required exchange with oxygen and also provides better resistance against potential corrosion. Innosoft B580 fully removes any residues left following the use of Innosoft B570. It passivates the surface, preventing further corrosion.

Innosoft B570 has deep-cleaning properties, which reduce the risk of 'under deposit corrosion' of stainless steel. This type of corrosion occurs due to limited oxygen access. This is especially useful in chloride environments like marine applications. This phenomenon can also occur in polished or ground surfaces.

Although Innosoft B570 was developed for stainless steel, it also provides excellent results on other metals including aluminum, copper and plain steels. This product can be used to remove common corrosion pits on aluminum.

Instructions for Using Innosoft B570

First, Innosoft B570 should be shaken well and applied undiluted with a damp pad or sponge. In the case of polished or ground finishes, it should be applied in the same direction as the grinding, brushing, abrading or polishing.

The oxide or loosened dirt may cause slight scratching, especially on polished surfaces and should be removed with a damp cloth or water. Only mild pressure should be used in this case so that dirt does not lodge in surface discontinuities.

This can be followed with the basic cleaner Innoclean B560, which is not only neutral but also leaves a passive layer, reducing the risk of any further corrosion.

Waterproof gloves can be worn while using the Innosoft B570 product. A Material Safety Data Sheet corresponding to 1907/2006/EC article 31 is available on request.

Instructions for Using Innoprotect B580

Innoprotect B580 is a neutral (pH 6.5) and mild cleaner that can be used along with Innosoft B570. This product can remove loosened contaminants and oxides, and also passivates the surface to reduce further corrosion.

It is perfect for daily cleaning of lightly soiled surfaces that have been already treated with Innosoft B570.

Innoprotect B580 can be used to remove dirt and oxides released by previous usage of Innosoft B570. All trace of contamination can be removed through the generous use of the diluted cleaner (400 ml in 1000 ml of cold water).

Post drying if a light shading is seen, it is obvious that some residues are lodged on the surface or in discontinuities. The cycle should be repeated until the surface is fully cleaned.

This information has been sourced, reviewed and adapted from materials provided by Aalco - Ferrous and Non-Ferrous Metals Stockist.

For more information on this source, please visit Aalco - Ferrous and Non-Ferrous Metals Stockist.