Stainless steel is supposedly impervious to discolouration, tarnishing and rusting. Therefore, when external stainless steel architectural fittings, such as planter edging, corrodes it creates problems and confusion. Surely, as its name suggests, stainless steel should not corrode?
Stainless steel is best described as corrosion resistant rather than corrosion free. In most environments, particularly indoors, it is corrosion free but it if exposed to highly corrosive elements such as chlorine, it will corrode albeit at a slower rate than carbon steel.
How is does stainless steel protect against rust?
All stainless steel has at least 12% chromium content. It is this chromium, not present in carbon steel, that is crucial to keeping stainless steel from rusting.
Chromium works by reacting with oxygen in the air to produce a thin protective chromium oxide layer on the surface of the stainless steel. With carbon steel, the iron content in the steel would react with oxygen to form an iron oxide layer, which is commonly known as rust.
Therefore, with stainless steel the chromium oxide layer seals the iron content in the steel off from oxygen, giving the rust resistant properties associated with stainless steel.
If the chromium content is below 12%, the protective chromium oxide layer will not form.
The effect of Location on stainless steel corrosion:
When we come across a case of corroded stainless steel, the first thing we check is always the location. This is because in most corrosion cases, the type of corrosion is called tea staining, which is a brown discolouration on the surface. Tea staining is primarily caused by exposure to salt, and therefore is a problem to be aware of anywhere up to 20kms inland from the coast given the constant salt content in the air. It also occurs on roadsides because of salt spreading in winter.
Unlike rust, tea staining does not pose any risk to the structural integrity of the steel, but it is very unsightly. The best way to protect against tea staining is to use a higher grade of steel such as 316 grade which has additional alloyed elements within it to protect against the chloride ions in salt which are the cause of tea staining.
The effect of Grades on stainless steel corrosion:
As noted above, using a high-grade stainless steel such as 316 will give protection against tea staining.
Another common, and more cost effective, stainless steel grade is 304. Take care when specifying 304 grade stainless steel because it is more likely to corrode than 316. If specifying external architectural metalwork, such as wall top seat armrest, 304 grade is fine to use in an inland or sheltered location such as underneath a cycle shelter.
However, in environments where salt is present, 304 grade stainless is particularly susceptible to a form of corrosion called ‘pitting’. Essentially, pitting is the same as tea staining given that it is caused by chloride ions from salt. However, it is a more serious problem than tea staining because the corrosion can spread beneath the protective chromium oxide layer which can cause structural damage to the stainless steel in the same way that rust will damage carbon steel.
As a rough guide, the higher the chromium content of the stainless steel, the less likely it is to rust.
For more information on steel grades, please read our guide here.
Effect of maintenance (or lack of) on stainless steel corrosion:
Alongside selecting the correct grade for application, regular maintenance is the best way to protect against corrosion. Maintenance can either be proactive or reactive.
Proactive maintenance simply involves washing the steel down with a soap and water solution. This is only so effective in protecting against corrosion in harsh marine environments because the air is enriched with salt. To protect stainless steel against corrosion in marine environments, you would have to wash it as regularly as you wash your windows. Obviously, this is rather impractical so in these situations, we would strongly advise that you use a higher grade stainless steel.
Reactive maintenance involves using a commercial cleaner to effectively remove the protective chromium oxide layer, then washing down the steel with a soap and water solution. Although this may seem a little counter-intuitive, the stainless steel will quickly form a new chromium oxide layer which will not be contaminated by any corrosion.
Although stainless steel can corrode if placed in harsh environments or suffers mechanical damage such as scratches, the beauty of stainless steel is that the protective chromium oxide layer will form again given proper cleaning and the necessary chromium content (both of which 304 and 316 grade will have).
Unfortunately, stainless steel will not guarantee rust protection. It will guarantee increased protection against corrosion when compared to carbon, or mild steel, but in some cases it can and will corrode. We recommend specifying a higher grade whenever there is an element of doubt, and if possible, implementing a regular maintenance program to clean any exposed external architectural fittings.