Metallic Corrosion, Including Stainless Steel Corrosion Technical Data by Stirlings Australia

Topics Covered

Background

Forms of Corrosion

General Corrosion

Galvanic Corrosion

Erosion/Abrasion Corrosion

Intergranular Corrosion

Pitting Corrosion

Crevice/Shielded Corrosion

Microbiologically Induced Corrosion (MIC)

Stress Corrosion Cracking (SCC)

Corrosion Technical Information

Background

Stirlings Australia Global Metals Distributor, is located in Perth, Western Australia, and is an stockist distributor of stainless steel long, flat and fluid products and facilitator of value-added services. Established in 1986 as Stirling Metals, we're recognised as a leading stockist for quality stainless steel and copper nickel products. We're committed to providing innovative product and supply solutions to cater to each of our customer's stainless steel needs.

Stirlings Australia's success has been a result of years of building strong relationships and expert understanding within the industries we service. The main industries we supply to are; Architecture, Building & Construction, Food and Beverage, General Fabrication/Engineering, Marine and Shipbuilding, Resources and Construction and Wine.

As a continually improving and customer focused company Stirlings Australia is driven by a well-integrated system of people, technology, and innovative business approaches. We strive to make doing business with us easy for every customer, near or far, big or small, new or old. We believe in employing strategies such as enabling, making and delivering on promises. Most of all, we believe in our vision, aiming to be a world leader in providing our customers with excellent value, top quality products & outstanding service.

Stirlings Australia also has a dedicated team of project consultants, specialising in project supply management. With expertise in customising supply requirements to meet specific project needs and deadlines, we also pride ourselves in delivering felxible, reliable and efficient service with cost-effectiveness in mind. Supported by an established network of international partners and logistics companies, we act as your company's stainless steel supply division, working with you to ensure the success of your project.

Forms of Corrosion

Stainless steels are not indestructible materials, nor immune to all corrosive attack. However, the family of stainless steels are excellent combatants of corrosion and provide a wide choice of materials which, with careful selection and proper fabrication, enable the manufacture of cost effective critical components to meet the diverse needs of many industry sectors.

The more common forms of corrosion which affect all metals and alloys, including stainless steel, are briefly outlined.

General Corrosion

General corrosion is a uniform attack of the entire metal surface. It is the least dangerous because rates of corrosion can be measured and predicted. Stainless steels have very low general corrosion rates in many aggressive environments.

Galvanic Corrosion

Galvanic corrosion occurs when two different metals are in electrical contact and immersed in the same corrosive solution. Stainless steels are noble metals and therefore seldom suffer increased corrosion rates as a result of galvanic corrosion.

Erosion/Abrasion Corrosion

Erosion/abrasion corrosion is a combination of mechanical and corrosive attack. Abrasive particles in suspension, or high velocities, expose fresh metal surfaces which then suffer high rates of corrosion. Stainless steels offer a high resistance due to the tenacious and stable passive film on their surface.

Intergranular Corrosion

Intergranular corrosion is due to the formation of chromium carbides at high temperatures (450°-859°C).These form preferentially at the grain boundaries thus reducing the chromium content and resulting in a path of lower corrosion resistance around the grains. With correct choice of material (“L” or stabilised grades) and care during fabrication this form of corrosion should not occur.

Pitting Corrosion

Pitting corrosion is a dangerous, very localised form of corrosion which results in small holes or perforations through the material, but with little measurable general metal loss. Some corrosive environments (commonly those containing the aggressive chloride ion) have the ability to attack localised weak points in the passive film. Due care in the selection of material should be taken if aggressive (eg chloride) ions are present, especially in acidic solutions at elevated temperatures. If conditions which promote pitting corrosion cannot be modified, materials with higher alloy content such as the duplex stainless steels and the stainless alloys will often give a solution to the problem.

Crevice/Shielded Corrosion

Crevice/shielded corrosion occurs where the surface of stainless steels is shielded or occluded thus preventing the free access and availability of oxygen to the surface.The passive film therefore tends to break down in these areas. Any conditions which give rise to a “crevice” should be avoided.

Microbiologically Induced Corrosion (MIC)

MIC results from the attraction and adherence of bacteria to the surface of the metal. A condition similar to a crevice is thereby produced. Certain bacteria produce aggressive metabolic products which aggravate the situation.

Stress Corrosion Cracking (SCC)

Both pitting and crevice corrosion can lead to SCC under certain conditions. Stress corrosion cracking is a brittle fracture occurring in an otherwise ductile material.The austenitic crystal structure is prone to SCC whereas the ferritic crystal structure prevents its development.

SCC requires the following three factors for development:

The presence of tensile stress.This can be either applied or residual stress as occurring as a result of the metal forming, fabrication and welding procedure.

A minimum temperature – generally SCC does not occur under 60°C.

The presence of a particular ion, eg. the chloride ion. It is often difficult to quantify the exact chloride concentration needed, but localised concentration of chlorides will often initiate SCC.

These three factors are synergistic to a degree. SCC is a process whereby initiation may take considerable time. However, once the pit has developed it acts as a local stress raiser, the pit yawns open and fresh electrolyte reaches the anodic tip of the pit. Corrosion occurs locally at the tip of the pit where further yawning and possible branching occurs.

Eventually, the progress of attack reaches such a degree that the yield stress of the material is exceeded due to the residual of sound material.

High resistance to SCC is obtained by use of duplex stainless steels, stainless alloys and super ferritic stainless steels.

Corrosion Technical Information

Technical information on the resistance of stainless steels, molybdenum stainless steel, duplex stainless steel and 3CR12/5CR12 with chemicals at different temperatures are as follows:

Table 1. Corrosion technical information

R indicates the material is resistant to the named chemical up to the temperature shown, subject to limitations indicated by the footnotes.
NR indicates that the material is not recommended.
ND indicates that no data is available.

1. not if chlorides present
2. limited data
3. depending upon the acid
4. acid fumes dry, attack may occur if moisture builds up
5. anhydrous
6. depending upon concentration
7. may discolour with time
8. in strong solutions only when inhibited
9. pitting possible in stagnant conditions
10. possibility of pitting NR indicates that the material is not recommended.
11. may cause stress corrosion cracking
12. when free of SO₂
13. may cause contamination of product
14. dilute hypochlorites can be used to sterilise some stainless steel with extreme care
15. general corrosion may become excessive
16. 10%
17. in the absence of impurities
18. dilute
19. some attack at high temperatures

Source: Stirlings Australia

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