Corrosion – Part one
This is the first in a series of four blogs. We will look first at types of corrosion and the steps we take to minimise them, then in the next blog, the factors (particularly chemistry) that affect a metals’ ability to withstand corrosion. The third blog will discuss what corrosion tests are available and what each test actually looks for, and finally blog four will review how our stock grades rank in terms of corrosion resistance.
The starting question is “does corrosion resistance in steels matter”? Pitting and crevice corrosion at higher temperatures (e.g. above 50˚C to 60˚C) and stress corrosion cracking (SCC) are the most common of all types of corrosion in oil and gas applications. The worst is caused by localized attacks from seawater, where the chloride content, exacerbated by fouling and galvanic effects, induce stress corrosion cracking. Corrosion equals reduced component, waste and cost.
But what is corrosion? Do a Google (other search engines are available!) search and you will come up with 3 (at least) definitions:
- Corrosion is a chemical or electromechanical reaction between a material and its environment that produces a change of the material or its properties – most recognisable as the formation of iron oxide on the outside of an iron bar or copper turning green.
- The chemical elements in the corroding material are trying to go back to the most stable (pre-production) condition it was once in – i.e. its ore-like condition using the thermo-reaction it has with its surrounding environment.
- Corrosion is an electrochemical process where metal atoms on the surface of the metal dislodge from the rest of the solid and enter the environment as a positive ion and they then combine with oxygen & hydrogen in the environment to form an oxide layer.
Will that help you do your job better? Probably not.
Instead, let us get back to a basic level and look at the different types of corrosion and how the different types can be affected by our interaction with them:
Uniform corrosion – a corrosion process where thinning is uniform and proceeds without appreciable localised attack. This is the most common type and everyone will have experience of rust, or copper turning green. It particularly affects weathering steels, magnesium alloys, zinc alloys & copper alloys. Thus how we handle susceptible materials is very important, particularly if we store material in wet conditions, or we let iron comes into contact with a susceptible material. How could this happen? Well, every time we move, store and cut steel for example! So where there is a risk of contamination we have lined our racking arms and backs with wood to prevent metal-to-metal contact, and have also fit and use rubber sheaths on our fork lift trucks when transporting materials around the warehouse.
Stress Corrosion Cracking – the combined effect of tensile stress and a specific corrosive influence. Fine cracks appear at the attack site. Chloride stress cracking of stainless steels and ammonia stress cracking of nickel-copper alloys are examples. As we provide bar material, there is little we can do to prevent this in the final component – except supply fit for purpose and high quality material.
Intergranular corrosion – this is technically the dissolution of specific grain-boundary phases, usually with slight or negligible attack on the main body of the material. This type of corrosion usually results from composition changes at grain boundaries from elevated temperature exposure, i.e. after heat treatment. A commonly encountered form of intergranular corrosion is the attack of non-stabilized austenitic stainless steels due to the formation of chromium carbide precipitates and the subsequent depletion of chromium. While again we cannot cause this type of corrosion directly, we are aware of the causes and effects and pay particular attention to the quality of steel we provide and the metallographic properties required after any heat treatment.
Pitting – very localised corrosion and characterised by sharply defined holes in the metal that might eventually work their way through the material There may only be a small weight loss but the holes may grow rapidly. This corrosion normally occurs in stagnant and chemically laden environments – adjectives not usually applied to our warehouse! High concentrations of metal chlorides develop within the pit and hydrolyse to produce an acidic pH environment (having a high salt content & low oxygen concentration). The reactions within the pit become self-sustaining ultimately causing penetration through the metal. Every engineering metal (including stainless steels) is susceptible to pitting, but again we are aware of what to look for – for example when material arrived in 2013 from a mill which had contaminated their metal with iron residues there was a distinct possibility of this type of corrosion occurring so swift and decisive action was taken immediately.
Crevice corrosion – where pitting occurs in slots and gaps in metal-to-metal and metal-to-non-metal joins. Corrosion can be very rapid – stainless steel sheet can be cut in two by wrapping a rubber band around it and then immersing the sheet in seawater or dilute ferric chloride solution. Luckily we do not have many rubber bands on the shop floor, nor many areas that could be affected by crevice corrosion – unless you count the building joints, roof joints, steel work in the warehouse, rack arms, the truck, our cars etc!
Hydrogen embrittlement – corrosion takes place due to the penetration of the surface of susceptible metals by hydrogen. This can result from the formation of metallic hydride compounds in some materials while in others it takes place by the interaction of dissolved hydrogen (in water for example). Hydrogen embrittlement results in the formation and propagation of fine cracks and voids in the metallic structure. We are particularly careful that susceptible material, if received during wet conditions, is allowed to dry and not “trap” water within bundles.
Erosion Corrosion – Erosion corrosion is the increase in the rate of deterioration or attack on a metal because of the relative movement between a corrosive element and the metal surface, usually associated with wear and tear or the abrasive effect of two surfaces moving against each other. Again we are aware of the need to handle carefully and avoid metal-to-metal contact in susceptible metals.
Galvanic Corrosion – Galvanic corrosion results from the electrical coupling of two dissimilar metals (structure or chemical composition) in a corrosive medium resulting in the attack on the less resistant metal. The less “noble” material becomes anodic while the more noble becomes cathodic. The anodic material actually protects the cathodic leading to its own accelerated decay. Ranking of materials in an electromotive force (EMF) or galvanic series in a specific media will help determine the propensity of the two materials to this type of corrosion.
So we are constantly on guard against “rusty” material, or the possibility that our material could become corroded and constantly think and identify what steps we can take to minimise future problems for our customers……. and when we have a free moment play around with cutting some stainless sheet in half – all in the name of science of course!
If you have any questions on the above, please contact our Special Process Department on +44 114 232 9241. Next time we will look at what it is that attacks metal and the factors (particularly chemistry) that affect a metals’ ability to withstand corrosion.
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