Types of Corrosion Flashcards
How can corrosion cells be set up
Corrosion Cells, can be set up by many different means, but they all operate because there is some dissimilarity between the anode & the cathode.
Dissimilar metals.
Dissimilar phases in the metal grains.
Dissimilar energy levels. Dissimilar ion concentrations.
Dissimilar oxygen concentrations.
Explain dissimilar metal corrosion
Dissimilar Metal Corrosion (Galvanic Corrosion).
When two dissimilar metals are placed in an electrolyte a potential difference ( voltage) exists between them.
Demonstrated by placing two rods of different metals in water & connecting a voltmeter between them.
The voltmeter measures a voltage & current flows from the anode to the cathode via the outside connection.
The cell acts as a very basic, low powered battery.
How do you determine which metal becomes the anode and which the cathode
To determine which of the two will be the
cathode & which the anode, refer to the Galvanic Series.
Metals found lower in the series are anodic to
those above them.
Zinc is lower than steel; if zinc is connected to
steel in seawater, zinc will be the anode &
steel the cathode.
Concentration Cell or Crevice Corrosion
Concentration Cell or Crevice Corrosion
Associated with crevices in the order of 25 to 100µ wide
how do Concentration Cells occur
Initially, corrosion occurs uniformly over the surface, producing positive metal ions and negative hydroxyl ions
This consumes the dissolved oxygen, which results in the diffusion of more oxygen from the electrolyte
The oxygen in the bulk of the electrolyte is replaced more easily than that trapped in the crevice
This creates a low oxygen situation in the crevice, which impedes the
cathodic process, reducing Hydroxyl ion production
Excess positive ions accumulate in the crevice
This causes negative ions to diffuse there from the electrolyte
This is to maintain minimum potential energy overall
The metal ions, chloride & water molecules all react in complicated chemical reactions forming complex ions, in a hydrolysis reaction, resulting in corrosion products
Described by the equation
M+ + H2O => MOH + H+
The increase of Hydrogen ion concentration speeds up the corrosion
This increases the Chloride concentration
Which initiates an autocatalytic process
The electrolyte in an active crevice becomes very acidic, due to the increase in Hydrogen
The metal within the crevice corrodes, while that outside is cathodically protected
How does Pitting Corrosion occur
Pitting Is localised corrosion that selectively attacks areas of a metal surface
The corrosion process initiates uniformly on the surface of the steel under the water, which consumes oxygen This causes an oxygen gradient to form within the drop
Hydroxyl ions generated in the edge of the drop, diffuse inwards & react with Fe ions diffusing outwards, causing deposition of corrosion product around the pit.
This further retards the diffusion of O2 & accelerates the anodic process, making it autocatalytic.
The corrosion products accumulate over the pit forming a scab & isolating the environment within.
Wetted area around the water/air interface has more oxygen diffusion from the air than the centre of the drop This concentration gradient anodically polarises the central region, which dissolves
how does Intergranular Corrosion occur
Intergranular Corrosion Occurs between the grain boundaries in a material
because of intrusions in these regions.
Grain boundaries are preferred sites for the precipitation & segregation processes which occur in many alloys.
These intrusions are of two types:
Intermetallics - Compounds that are formed from metal atoms & having identifiable chemical formulae.
Can be either anodic or cathodic to the parent metal.
Compounds - are formed between metals & non- metallic elements, such as; Hydrogen, Carbon, Silicon, etc. Iron Carbide & Manganese Sulphide, are both important constituents of steel - both are cathodic to Ferrite (iron).
Any metal that has intermetallics or compounds at grain boundaries, will be susceptible to Intergranular Corrosion Plain carbon steel is a two-phase metal, some grains are cathodic, others anodic & Intergranular corrosion initiates
explain Grain Boundary Corrosion
The driving force behind Grain Boundary Corrosion is the area of higher energy found at the grain boundary.
These higher energy regions become Anodic sites.
The bulk of the grain itself becomes the Cathode.
This results in the loss of material in the Anodic reaction at the grain boundaries in the form of a line.
Weld Decay or Preferential Corrosion is an example of this type of corrosion.
The boundary is the fusion boundary that forms along the toe of the weld & is a region of higher energy.
This region becomes the anode & corrosion sets in.
What is Stress Corrosion (SCC)
Stress Corrosion Cracking. SCC
The combined action of stress & a corrosive environment, which leads to the formation of a crack, which would not have developed by the action of the stress or the environment alone.
The conditions necessary for SCC to occur are:
1) Tensile stress.
2) Suitable environment.
3) Sensitive metal.
4) Appropriate temperature & pH range.
SCC has the appearance of brittle mechanical fracture
Problems with pipes & tubes are common because of the hoop residual stresses
Stress-relieving heat treatments are a vital part of the QC
Explain Corrosion Fatigue
Corrosion Fatigue.
This has many similarities with SCC.
The most significant difference is:
*Corrosion fatigue is under Dynamic Stress
*SCC is under Static Stress
Fatigue affects all metals, causing failure at stress levels well below the UTS.
(UTS - Ultimate Tensile Stress)
Corrosion Fatigue can occur:
In any of the 3 states of the Pourbaix Diagram.
At stress levels much lower than those for SCC.
SCC growth rates are independent of the Kt factor
Fatigue crack growth is always effected by it.
Use of CP Systems that, over time, cause calcareous deposits to form, tend to inhibit crack growth.
How does Erosion Corrosion occur
Erosion Corrosion:
Deterioration that results from a metal being attacked because of the relative motion between a fluid & a metal surface, which accelerates the corrosion.
This type of corrosion is due to mechanical effects:
Wear – Abrasion – Scouring.
Two main forms of erosion corrosion are:
Laminar (fluid) flow & Impingement
Laminar flow may cause several effects:
The increased flow replenishes aggressive ions, (chloride & sulphide) which increases corrosion.
If the flow contains solid particles, protective layers may be scoured away causing corrosion.
In pipes, it prevents build-up of silt, preventing differential-aeration cells forming.
More O2 is carried to the area, minimising the formation
of differential-aeration cells. Supply of inhibitor is concentrated within the flow.
Turbulent Flow is much more straightforward:
Fluid molecules impinge directly on the metal, causing wear, which increases the corrosion rate.
Occurs inside a pipe because of turbulence caused by sudden changes in bore diameter, direction, welds or silt deposits.
Streamlining avoids turbulence reducing erosion corrosion
What is Fretting Corrosion
Fretting Corrosion: #Occurs at contact areas between materials under load, subjected to vibration & slip. #Appears as pits & grooves in the metal surrounded by corrosion products. #It’s detrimental due to destruction of metallic components & production of oxide debris.
Fretting Corrosion requirements:
1) An interface under load.
2) Repeated motion between the interfaces.
3) Could occur in the metal adjacent to clamps of risers or collars of conductors & caissons.
what is Biological Corrosion
Biological Corrosion:
- Also termed Microbiologically-Induced Corrosion (MIC) due to the effect of living organisms (bacteria).
- Sulphate-reducing Bacteria (SRB) is one of many types of bacterium, which can be found in pumping machinery, storage tanks, water-injection lines, etc.
- SRB metabolise sulphates & produce Sulphuric Acid or H2S, resulting in pitting or (SSCC) Sulphite Stress Corrosion Cracking
Biological Corrosion can be initiated by:
1) Production of corrosive substances like H2S or Ammonia, which result in direct chemical attack on the metal.
2) Producing, or being, a catalyst in the corrosive action.
3) Formation of concentration cells under the organisms.
4) Causing fretting due to the continuous movement of hard shelled creatures on the metal surface.
5) The reaction of SRB under anaerobic conditions.
The most important bacteria is, Sporovibrio desulphuricans.
- Thrives in the anaerobic conditions created under heavy accumulations of marine growth, thick deposits of corrosion products or mud.
- Because O2 is unable to diffuse through the heavy marine growth, the effect of this organism is to take the place of O2 in the usual cathodic reaction.
Factors Affecting Corrosion Rate
The Corrosion Rate is predictable within certain parameters & corrosion engineers work this out when designing a protection system.
There are, however, environmental factors that affect the overall corrosion reaction:
1) Temperature.
2) Water Flow Rate.
3) The pH of the Water.
How does temp increase corrosion
Temperature.
1) Most chemical reactions are speeded up by an increase in temperature.
2) Hot risers, exhaust & cooling-water dumps are all sites that can, & do, corrode more quickly than other sections of offshore structures.
3) These components being more susceptible, must be inspected more regularly.
