Corrosion principles pdf

As was explained in the previous Section, an electrical circuit is formed between the cathode and anode, thus allowing a potential difference to be established between them. It is this shift in potential as the corrosion current flows that is termed polarization. The effects of polarization are also important in galvanic bimetallic corrosion, which is more fully discussed in Section 8.

To illustrate this, consider the experimental copper-zinc cell shown in Attachment 4b. If the circuit is open and no current is flowing, then the electrodes each have a characteristic potential as shown in Attachment 5. Attachment 6a shows a plot of those potentials at zero current on a potential versus current graph. As more current is allowed to flow, the potentials continue to change polarize and the potential difference between the two decreases to a value limited primarily by the resistance of the electrolyte solution Re.

If polarization is not taking place, then there is no corrosion. The magnitude of strength of a corrosion cell is determined by the difference in electrical potential between the anode and the cathode. This voltage difference is greatly influenced by the level of polarisation which may be achieved in the corrosion circuit.

The passage of an impressed direct current will also have an effect on the level of polarization. The corrosion rate may rise or fall, depending upon the direction of the current. In the absence of oxygen, hydrogen deposited on cathodes as the result of cell action refer back to Attachment 4a tends to remain as a polarizing film that resists additional hydrogen ions. Resistance is due to the metallic path, the electrolyte path and the polarization of the anode and the cathode. This occurs when hydrogen is removed from the cathode.

That rate is largely controlled by the rate at which oxygen reaches the cathode. In oilfield operations, hydrogen removal from the cathode is usually accomplished by one of the mechanisms discussed in paragraph 4. The reason certain areas of the metal surface act as anodes centers on the inhomogeneities in the metal surface, in the electrolyte or both.

Potential differences on the metal surface are a natural result and are a primary cause of the initiation of localized corrosion cells. The type of corrosion that occurs usually gives a clue as to the major cause. Commercial metals are not homogeneous but contain inclusions, precipitates and several different phases.

Steel is an alloy of iron and carbon. Where pure iron is a relatively weak, ductile material, the alloy, steel, is a much stronger material. As a result of combining part of the iron with carbon, the resultant metal contains dissolved carbon and precipitated iron carbide Fe3C.

The iron carbide that has a lower tendency to corrode than pure iron is distributed within the iron as tiny microscopic islands. The Fe3C and pure iron are in electrical contact so when the steel is placed in an electrolyte, an electrical circuit is completed and current flows through thousands of microcells on the steel surface.

The locations of the anodic and cathodic areas of the microcells may change as corrosion products accumulate shifting the anodic areas. In solid solution alloys, such as or stainless steel, or Monel , there may be potential differences arising from concentration differences from point to point. This concentration difference can be evident when metals are formed or reformed such as in castings or weldment of metals. Here is where an area relationship can be very harmful.

If the anode is small compared to a large cathode, the corrosion may occur at a rapid rate due to the high current density on the anode. Local heating, such as welding, can result in changes of the nature of phases and their compositions or changes in grain size and grain boundaries creating differences in potentials.

Intergranular attack may be accelerated by the potential differences between the grains and grain boundaries. Stress such as that applied during cold working or cold forming of the metal can set up potential differences along the surface of the metal.

The stressed areas will be anodic to the unstressed areas. Scratches or abrasions on the metal surface will act in the same manner. The first is the conductivity of the electrolyte and the effect of electrolyte on the base corrosion potential of the system.

The second has to do with the presence or absence of oxidizing agents which are necessary for the cathodic portion of the corrosion cell. The anodic reaction cannot occur in the absence of a corresponding reaction at the cathode, regardless of the conductivity of the cathode. The more conductive the electrolyte, the easier current can flow and the faster corrosion will occur.

The amount of metal that dissolves is directly proportional to the amount of current flow between anode and cathode. For iron, one amp of current flowing for one year will result in the loss of 20 pounds 9.

It is important to remember that other factors will also have an impact on the corrosivity of the electrolyte, the conductivity only determining the ease at which corrosion currents are able to flow from anode to cathode. Hydroxyl ions OH- make a solution basic or alkaline and force the pH towards The actual variation of corrosion rate with pH is dependent on the composition of the electrolyte.

Since pH is a negative logarithmic function, a fold reduction or increase in the hydrogen ion concentration is required to effect a pH change of one. Dissolved gases are the primary cause of most corrosion problems in oil and gas production.

The following paragraphs discuss each gas independently, but it is important to note that corrosion rates are also greatly influenced by physical variables such as temperature, pressure and velocity. Similarly, the Figures referred in these paragraphs are for specific conditions and are only intended to reflect the relative corrosion tendencies of each. Attachment 7 is a composite graph from results of three different studies showing corrosion rates as a function of oxygen concentration: The solubility of oxygen in water is a function of pressure, temperature, and chloride content.

Water from lakes, streams, fresh water aquifers, rain or oceans usually will be oxygen saturated. Oxygen is more soluble at high pressures and lower temperatures and is less soluble in salt water than in fresh water. Oxygen accelerates corrosion under most circumstances because it is a strong and rapid oxidising agent in cathodic reactions.

It will easily combine with electrons at the cathode and allow the corrosion reactions to proceed at a rate limited by the rate at which oxygen can diffuse to the cathode as discussed in Section 4. Corrosion in the presence of dissolved CO2 is referred to as sweet corrosion. Increased pressure, reduced temperature, or reduced water salinity each increase CO2 solubility which lowers pH.

Many dissolved minerals buffer the water, thus minimising the effects of the above changes on pH reduction. Partial pressure of carbon dioxide can be used as a yardstick to predict the corrosiveness of a system.

Attack due to the presence of dissolved hydrogen sulphides is referred to as sour corrosion. Hydrogen sulphide can be generated by sulphate reducing bacteria SRB. These bacteria contribute to corrosion by their ability to flourish in the absence of oxygen and their ability to change sulphate ions into hydrogen sulphide.

The anaerobic conditions under a colony constitute a differential aeration cell with the bulk of the electrolyte, whereas their ability to produce hydrogen sulphide can cause severe localized corrosion. Refer to Section 8. Under certain pressure conditions, the hydrogen produced by the corrosion reaction can diffuse into the metallic lattice to cause embrittlement and subsequent cracking of susceptible metals.

These reactions, therefore, reduce polarization by allowing more cations to come into solution that increases the conductivity of the electrolyte. See Section 8. Correct application inhibitors and cathodic protection as corrosion control methods are very dependent on these variables. Temperature and pressure are interrelated, and the corrosivity of a system is further influenced by velocity. For example, in a system open to the atmosphere, the corrosion rate generally increases with increasing temperature until the concentration of dissolved gases decreases.

In a closed system, this is not necessarily the case. More gas goes into solution as the pressure increases, which may, depending on the dissolved gas, increase the corrosivity of the solution. Stagnant or low velocity fluids usually give low general corrosion rates, but pitting rates may be high.

Corrosion rates generally increase with increasing velocity due to the depolarising effect on the cathode. High velocities and the presence of suspended solids or gas bubbles can lead to erosion corrosion, impingement, or cavitation. On the other extreme, oil, gas, or multi-phase pipelines operating at low velocities can result in corrosion along the bottom of a pipeline.

The low flow condition is referred to as stratified or laminar flow which can be modelled using commercially available computer programs. This type of corrosion can be common in atmospheric corrosion of industrial or marine locations. Strong uninhibited acids will uniformly corrode steel very rapidly.

In general, however, uniform corrosion occurs gradually thus failures take longer to occur. Uniform corrosion can be addressed in original design criteria by adding a corrosion allowance to design metal thickness. It is very important to recognize that this is not a corrosion control measure. The extra metal thickness simply allows more time for detection and mitigation before equipment capability or operations safety is jeopardized.

Refer to Attachment 9 which lists the relative severity of average uniform corrosion rates. It occurs when the metal undergoing corrosion suffers metal loss at localized areas rather than over a large area or the entire surface area.

The entire driving force of the corrosion reaction is concentrated at these localized areas. The corrosion rate at these areas will be many times greater than the average corrosion rate over the entire surface. Pitting is much more dangerous than uniform corrosion because the pitted area can become penetrated in a short time.

Refer to Attachment 9 which lists the relative severity of corrosion pitting rates. In oxygenated systems, oxygen in the crevice may be consumed more rapidly than fresh oxygen can diffuse into the crevice. This difference in oxygen concentration at crevices and open areas creates a potential difference and results in corrosion of metal in oxygen-deficient areas.

In addition, the pH in a crevice decreases compared to the bulk solution which may result in a more acidic environment thus accelerating corrosion. This is particularly the case with austenitic and martensitic series stainless steels. An example is a bare or poorly coated pipeline buried under a paved or compacted road. Under the road, the soil is not well aerated, but on either side of the road the soil is well aerated.

Locally, severe attack can occur on the pipe at the edge of the road in the transitions from the compacted nonaerated soil to the loose, aerated soil. In addition, sulphate reducing bacteria may thrive under these deposits magnifying the corrosion process by creating hydrogen sulphide. When a corroding metal becomes covered with a corrosion product that is dense, impermeable and adherent, the product protects the metal from its environment. Localized removal of this protective scale may precipitate the onset of preferential corrosion of the exposed metal surfaces.

If the scale is a conducting scale e. FeS , then damage to it could result in intense attack of the area that lost its protective scale layer. This is the classic pit scenario.

In the case of detachment of a conducting scale, or removal of a non-conduction scale e. Stainless steels derive their corrosion resistance from the formation of a thin iron chromium oxide film and the ability to maintain that film. If the film becomes destroyed at local areas, those areas become anodic and pitting results. This coupling of dissimilar metals is referred to as a bimetallic couple or a galvanic cell.

It can be destructive, accelerating the corrosion rate of the more reactive of the two metals. When steel is connected to a more reactive metal, such as magnesium, the steel is the cathode and is protected; and the magnesium is the anode and corrodes. However, in a cell with a small anode and a large cathode, corrosion is concentrated at the anode and severe corrosion damage will occur.

Refer to Attachment 3. The fluid has to be corrosive to at least one member of the dissimilar metal couple for bimetallic corrosion to occur. This difference in microstructure can create a potential difference and set up a corrosion cell.

A similar problem can result from an improper choice of welding rods. In this case, the potential differences are due to the differences in the composition of the weld and parent metals. The heat required in upsetting causes the transition zone in the heated end, or upset, to have a different grain structure from the rest of the pipe body. Fully normalizing the tubing after the upset process will return uniformity to the grain structure which will eliminate this corrosion potential.

Removal of this scale at local areas can lead to accelerated attack. High velocity flow or turbulence can erode away the protective scale to expose fresh metal to corrosive attack. The combination of the erosion of the scale and corrosion of the underlying metal is termed flow enhanced corrosion erosion corrosion.

Carbon steel piping systems and downhole tubing must be designed to maintain velocities below the API RP 14E critical velocity discussed in Section 7. This occurs when a stream impinges upon a metal surface and breaks down protective films at very small areas. The resulting attack is characteristically in the form of pits elongated and undercut on the downstream end. Impingement often results from turbulence surrounding small particles adhering to a metal surface.

Cavitation is the formation and collapse of vapour bubbles in fluids because of rapid changes in pressure. It can occur whenever the absolute pressure at a point in the liquid stream is reduced to the vapor pressure of the fluid so that bubbles form, and this is followed by a rapid rise in pressure resulting in bubble collapse.

Intergranular corrosion can occur in the absence of stress. It is caused by precipitation of impurities at grain boundaries, or changes in the alloying elements at the grain boundary areas. For example, depletion of chromium in the grain boundary of stainless steel can be lead to intergranular corrosion. Some of the hydrogen atoms combine to form gaseous molecular hydrogen H2 on the metal surface and are released to the environment.

A portion of the atoms are absorbed by the metal and this entry of hydrogen atoms into the metal may have some very undesirable effects. Hydrogen- induced cracking HIC and hydrogen embrittlement are two types of phenomena that can occur.

A lamination or void in the steel provides a place for hydrogen atoms to combine, form hydrogen gas, and result in sufficient pressure to cause blistering. Blisters are visible on a surface when the voids responsible for the HIC are near the surface.

SWC is initiated at inclusions on different planes through the metal wall. The initial crack propagation occurs along the axis of the inclusion and then interconnects with cracks on other planes to form the SWC.

These stresses will cause cracks to initiate and propagate perpendicular to existing HIC. This can result in cracks that eventually propagate through the steel wall see Attachment 8. Namboodhiri Follow. Tablas de polaridad de solventes organicos. The holy geeta chapter moksha-sannyasa yoga.

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Elizabeth Howell. Principles of corrosion 1. Namboodhiri Retd. Professor, Inst. Of Tech. Negative potentials indicate spontaneous reaction.

Oxidation and reduction reactions are of equal rate. This process is polarization. Harini Gopalakrishnan Dec. HitheshAjith1 Mar. KarimAsaad4 Nov. Raja Prian Sep. Show More. Total views. You just clipped your first slide! Clipping is a handy way to collect important slides you want to go back to later. Now customize the name of a clipboard to store your clips. Visibility Others can see my Clipboard. Cancel Save.