Transcript Slide 1
Lecture notes on Chapt. 13 and 14 Corrosion Engineering
Corrosion Engineering is the engineering design of corrosion control methods and the solution of in-service problems, including: Corrosion prevention Material selection Paints and coatings Chemical treatments Corrosion results in billions of dollars. The problem is largely avoidable by proper maintenance and protection methods. Corrosion prevention and control programs can help reduce the high cost of corrosion.
Economics are a key consideration in corrosion engineering. Develop and implementation of corrosion prevention and control maintenance procedures Implement a monitoring program for early detection of incipient corrosion problems.
Reduce the impact of corrosion and develop cost-effective corrosion control programs A corrosion Engineer has to provide technical services in four areas: Prediction, Assessment and Diagnosis, Testing Remediation/Mitigation. A Corrosion engineer should be able to: Set up test programs, Analyzes information acquired from different sources Construct profiles of corrosion problems. Suggest operating or maintenance schemes, create test programs for selecting new materials or altering operating conditions, and devise remedial action plans for corrosion problems.
Classification of corrosion protection methods
Active corrosion protection Passive corrosion protection Permanent corrosion protection Temporary corrosion protection
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Active corrosion protection
The aim of active corrosion protection is to influence the reactions which proceed during corrosion, it being possible to control not only the package contents and the corrosive agent but also the reaction itself in such a manner that corrosion is avoided. Examples of such an approach are the development of corrosion-resistant alloys and the addition of inhibitors to the aggressive medium.
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Passive corrosion protection
In passive corrosion protection, damage is prevented by mechanically isolating the package contents from the aggressive corrosive agents, for example by using protective layers, films or other coatings. However, this type of corrosion protection changes neither the general ability of the package contents to corrode, nor the aggressiveness of the corrosive agent and this is why this approach is known as passive corrosion protection. If the protective layer, film etc. is destroyed at any point, corrosion may occur within a very short time.
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Permanent corrosion protection
The purpose of permanent corrosion protection methods is mainly to provide protection at the place of use. The stresses presented by climatic, biotic and chemical factors are relatively slight in this situation. Machines are located, for example, in factory sheds and are thus protected from extreme variations in temperature, which are frequently the cause of condensation. Examples of passive corrosion protection methods are: Tin plating Galvanization Coating Enameling Copper plating
Temporary corrosion protection
The stresses occurring during transport, handling and storage are much greater than those occurring at the place of use. Such stresses may be manifested, for example, as extreme variations in temperature, which result in a risk of condensation. Especially in maritime transport, the elevated salt content of the water and air in so called salt aerosols (salt spray) may cause damage, as salts have a strongly corrosion-promoting action. The following are the main temporary corrosion protection methods:
Protective coating method , Desiccant method and VCI method
1. Protective coating method
The protective coating method is a passive corrosion protection method. The protective coating isolates the metallic surfaces from the aggressive media, such as moisture, salts, acids etc..
The following corrosion protection agents are used:
Solvent-based anticorrosion agents
Very high quality protective films are obtained. Once the anticorrosion agent has been applied, the solvent must vaporize so that the necessary protective film is formed. Depending upon the nature of the solvent and film thickness, this drying process may take as long as several hours. The thicker the film, the longer the drying time. If the drying process is artificially accelerated, there may be problems with adhesion between the protective film and the metal surface.
Since protective films are very thin and soft, attention must always be paid to the dropping point as there is a risk at elevated temperatures that the protective film will run off, especially from vertical surfaces. Since solvent-based corrosion protection agents are often highly flammable, they may only be used in closed systems for reasons of occupational safety.
Water-based anticorrosion agents
Water-based anticorrosion agents contain no solvents and thus do not require closed systems. Drying times are shorter than for solvent-based anticorrosion agents. Due to their elevated water content, water-based anticorrosion agents are highly temperature-dependent (risk of freezing or increased viscosity). The advantage of this method is that the protective film is readily removed, but the elevated water content, which may increase relative humidity in packaging areas, is disadvantageous.
Corrosion-protective oils without solvent
Corrosion-protective oils without solvent produce only poor quality protective films. Good quality protection is achieved by adding inhibitors. Since these corrosion-protective oils are frequently high quality lubricating oils, they are primarily used for providing corrosion protection in closed systems (engines etc.).
Dipping waxes
T he protective layer is applied by dipping the item to be packaged into hot wax. Depending upon the type of wax, the temperature may have to be in excess of 100 °C. Removal of the protective film is relatively simple as no solid bond is formed between the wax and metal surface. Since application of dipping waxes is relatively complex, its use is limited to a few isolated applications.
2. Desiccant method Introduction
“Desiccant bags are intended to protect the package contents from humidity during transport and storage in order to prevent corrosion, mold growth and the like". The desiccant bags contain desiccants which absorb water vapor, are insoluble in water and are chemically inert, such as silica gel, aluminum silicate, alumina, blue gel, bentonite, molecular sieves etc. Due to the absorbency of the desiccants, humidity in the atmosphere of the package may be reduced, so eliminating the risk of corrosion. Since absorbency is finite, this method is only possible if the package contents are enclosed in a heat sealed barrier layer which is impermeable to water vapor. This is known as a climate-controlled or sealed package. If the barrier layer is not impermeable to water vapor, further water vapor may enter from outside such that the desiccant bags are relatively quickly saturated, without the relative humidity in the package being reduced.
Desiccants are commercially available in desiccant units.
"A desiccant unit is the quantity of desiccant which, at equilibrium with air at 23 ± 2°C, adsorbs the following quantities of water vapor: min. 3.0 g at 20% relative humidity min. 6.0 g at 40% relative humidity The number of desiccant units is a measure of the adsorption capacity of the desiccant bag." Desiccants are supplied in bags of 1/6, 1/3, 1/2, 1, 2, 4, 8, 16, 32 or 80 units. They are available in low-dusting and dust-tight forms. The latter are used if the package contents have particular requirements in this respect.
Barrier films
Barrier films are available in various forms, for example as a polyethylene film or as a composite films with two outer polyethylene layers and an aluminum core. The composite film performs far better with regard to water vapor permeability (WVP), achieving WVP values of below 0.1 (g/m²d). In the composite film, the barrier layers are arranged so as to bring about a considerable reduction in permeability in comparison with a single layer.
In accordance with current standards, water vapor permeability is always stated for both 20 °C and 40 °C. According to information from the manufacturer, it may be concluded that water vapor permeability rises with increasing temperature and falls with increasing thickness. This problem occurs most particularly with polyethylene films, while aluminum composite films are largely insensitive to rises in temperature.
Placement of desiccant bags
The desiccants should be suspended from strings in the upper part of the climate-controlled package to ensure good air circulation around them. It is essential to avoid direct contact between the desiccant bag and the package contents as the moist desiccant would promote corrosion. It is advisable to use numerous small bags rather than fewer large ones, as this increases the available surface area of the desiccant and so improves adsorption of the water. In order to ensure the longest possible duration of protection, the barrier film must be heat sealed immediately once the desiccant bags have been inserted. Desiccant bags are always supplied in certain basic package sizes which, depending upon the desiccant unit size, may contain a single bag (of 80 units) or up to 100 bags (of 1/6 unit). The basic outer package should only be opened directly before removal of a bag and must immediately be heat sealed again.
Comparison of advantages and disadvantages of the desiccant method Advantages
Desiccants provide excellent corrosion protection to both metallic and nonmetallic items Removal of the desiccant on delivery to the receiver is straightforward, unlike the removal of protective films in the protective coating method. The package contents are immediately available. No particular occupational hygiene requirements apply as the desiccant is non hazardous.
Disadvantages
Placement of the desiccant bags and heat sealing of the barrier films are relatively labor-intensive. The slightest damage to the barrier layer may negate the effectiveness of corrosion protection. Calculating the required number of desiccant units is not entirely simple and it is easy to over-calculate. However, too much protection is better than too little. Humidity indicators inside the package are not very reliable as they are only valid for certain temperature ranges.
3. VCI (Volatile Corrosion Inhibitor) method Mode of action and use
Inhibitors are substances capable of inhibiting or suppressing chemical reactions. They may be considered the opposite to catalysts, which enable or accelerate certain reactions. Unlike the protective coating method, the VCI method is an active corrosion protection method, as chemical corrosion processes are actively influenced by inhibitors. In simple terms, the mode of action is as follows: the substance (applied onto paper supports or in a powder or spray formulation) passes relatively continuously into the gas phase and is deposited as a film onto the item to be protected. This change of state proceeds largely independently of ordinary temperatures or humidity levels. The inhibitor inhibits corrosion in the aggressive, corrosive medium, suppressing either the anodic or cathodic half-reactions. Under certain circumstances, the period of action may extend to two years.
The mode of action dictates how VCI materials are used. At item to be protected is, for example, wrapped in VCI paper. The metallic surfaces of the item should be as clean as possible to ensure the effectiveness of the method. The VCI material should be no further than 30 cm away from the item to be protected. Approximately 40 g of active substances should be allowed per 1 m³ of air volume. It is advisable to secure this volume in such a manner that the gas is not continuously removed from the package due to air movement. This can be achieved by ensuring that the container is as well sealed as possible, but airtight heat sealing, as in the desiccant method, is not required. The VCI method is primarily used for items made from steel, iron, nickel, chromium, aluminum and copper, for which it provides good protection. The protective action or compatibility of inhibitors with specific alloys must be clarified with the manufacturer. Ordinary commercial VCI materials provide no protection to zinc, cadmium, tin, tungsten or lead.
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Comparison of advantages and disadvantages of the VCI method
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Advantages
Since the gas also penetrates holes and cavities, these areas also receive adequate protection. The period of action may extend to two years. The wrapping need not be provided with an airtight heat seal. On completion of transport, the packaged item need not be cleaned, but is immediately available. • • •
Disadvantages
The VCI method is not suitable for all metals. It may cause considerable damage to nonmetallic articles (plastics etc.). Most VCI active substances may present a hazard to health, so it is advisable to have their harmlessness confirmed by the manufacturer and to obtain instructions for use.
Different Corrosion protection methods
Anodizing
Advantages
A tough surface layer which has very good corrosion protection properties and very good adhesion to the surface.
Disadvantages
Must be applied after welding or brazing if the joining areas are to be protected. This can be complicated for large structures. Can not be done on site.
When to use
For protection against weathering and scratching/abrasion.
Conversion coating
Advantages
A non costly and simple protection of the aluminum surface, which in addition increases the adhesion of lacquers and adhesives.
Disadvantages
Has limited resistance to mechanical and thermal influence.
When to use
Primarily used as a pre-treatment before lacquering or adhesive bonding. Lacquering
Advantages
A high quality lacquering system has very good corrosion protection properties.
Disadvantages
The performance of the lacquering system is very dependent on the quality of the pre-treatment and application work. Relatively expensive.
When to use lacquering
Where appearance and/or corrosion performance is very important
Inhibitors
Advantages
Can be tailored to give excellent protection in specific environments.
Disadvantages
Expensive to use with large amounts of liquid. May cause increased corrosion if incorrectly used.
When to use inhibitors
For protection against internal corrosion in closed systems, circulating or non-circulating Protective adhesive tapes
Advantages
Prevents galvanic contact. Grease filled tapes will seal crevices.
Disadvantages
Costly to apply. May need to be supported in place.
When to use
Buried pipelines
Application of CP Engineering and Design, Cathodic Protection
, requires both cathodic protection (CP) and coatings, regardless of soil or water resistivity, for the following buried or submerged ferrous metallic structures: • • • • • • • • • Natural gas and propane piping Liquid fuel piping Oxygen piping Underground storage tank (UST) systems Fire protection piping Steel water tank interiors Ductile or cast iron pressurized piping under floor (slab on grade) in soil Underground heat distribution & chilled water piping in ferrous metallic conduit in soils Other structures with hazardous products
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Schematic of an Impressed Current CP System
• In the impressed current CP, the large electrochemical is formed between an anode and the structure to be protected by a power supply that is controlled by reading a reference electrode close to the structure.
Impressed Current Cathodic Protection System
The system depicted above shows one way by which cathodic protection may be applied. In this system, power is drawn from the national grid and converted into a dc current by means of a transformer rectifier. This is not the only method by which the dc current which is required may be supplied. In remote areas, or parts of the world where a mains supply is not available, the driving force for the current is often provided by a diesel generator, solar cell, ..
Protection of underground tanks
• • The basic principle of cathodic protection (CP) is simple. A metal dissolution is reduced through the application of a cathodic current. Cathodic protection is often applied to coated structures, with the coating providing the primary form of corrosion protection. The CP current requirements tend to be excessive for uncoated systems. The
first application of CP dates back to 1824
, long before its theoretical foundation was established. Cathodic protection has probably become the most widely used method for preventing the corrosion deterioration of metallic structures in contact with any forms of electrolytically conducting environments, i.e. environments containing enough ions to conduct electricity such as soils, seawater and basically all natural waters.
Cathodic protection basically reduces the corrosion rate of a metallic structure by reducing its corrosion potential, bringing the metal closer to an immune state. The two main methods of achieving this goal are by either: • • •
Using sacrificial anodes
with a corrosion potential lower than the metal to be protected (see the seawater galvanic series)
Using an impressed current
provided by an external current source
Design Procedures
• 1. Area to be protected • 2. Polarized Potential • 3. Current Demand • 4. Anode Consumption • 5. Anode # and distribution • 6. Anode resistance • 7. design output current • Use FEM and Laplace Eqn:
d 2
V/
d
x2=0
Criteria for effective cathodic protection
To design, operate and monitor a cathodic protection system it is important to measure its effectiveness against well established protection criteria. Since CP involves the depression of the structure potential and the supply of electrons, invariably most of the protection criteria are based on either potential or current.
The potential criterion
From the basic electrochemical theory absolute protection (zero corrosion rate) is achieved if the structure is polarized to the reversible electrode potential of the anodic reaction. However, the determination of the reversible electrode potential by either calculation or experiment is impossible in environments of practical interest, since for most environments the ferrous ion concentration is not known and thermodynamics (the Nernst equation) cannot apply.
Field experience has shown that in aerated soils mild steel was fully protected at a potential of -850 mV vs. Cu/CuSO 4 (-800 mV vs. Ag/AgCl/seawater, +250 mV vs. Zn/seawater and -780 mV vs. SCE). It is also widely accepted that this potential value ensures complete cathodic protection of steel in aerated seawater at ambient temperatures. Under anaerobic conditions (for example in mud), it is recommended that the protection potential should be -950 mV vs. Cu/CuSO 4 in order to combat the increase in corrosion rate caused by microbial activities. It is important to note that the values quoted for the protection potential refer to the potential difference between the structure and the reference electrode without extraneous effects such as IR drop or field interference.
100 mV Shift criterion
This criterion requires that when the current is switched off, the instant potential shall be at least 100 mV more negative than the free corrosion potential. This criterion has seen some acceptance with the CP of rebar steel in concrete.
Potential 'swing' criterion
Another criterion based on field experience, is that a negative potential change of 200 - 300 mV from the free corrosion potential is a good measure of adequate protection.
Impressed current systems:
An alternative method of providing the current to protect a system is to use some sort of external power supply. As with the sacrificial system, the structure to be protected is made the cathode; the difference being that the driving force behind the current is not the difference in potential between the anode and cathode of the system but from the power supply. As the anode need not be less noble than the structure, the choice of materials is wider. Examples of different anode materials are: Platinum Titanium Graphite High Silicon Cast Iron
Stray Current Corrosion
Whenever a metallic structure is placed in the electric field between the structure and the anode, it provides an alternative route for the electron current path. Thus current can enter a foreign structure at one point and leave it at another location. At the interface on the foreign structure where the electrons move away from, corrosion is enhanced. This is known as stray current corrosion. This may be easily demonstrated in the laboratory and may be explained using a modified galvanic corrosion polarization diagram. Stray currents in soils could originate as well from DC electrified rail tracks. A pipeline buried nearby could suffer stray current corrosion. The influence of high AC voltage overhead power lines on the corrosion of nearby structures is subject to substantial investigation.
Stray current corrosion
refers to corrosion damage resulting from current flow other than in the intended circuit(s). For larger structures this term usually alludes to corrosion damage caused by extraneous current(s) flowing through soil and / or water.
Consequences of Overprotection
It is possible during cathodic protection to supply excess direct current to polarize a structure below the recommended protection potential. This state of affairs is termed 'overprotection'. There are two main consequences of overprotection, namely, waste of current and more seriously the violation of the structural integrity of the metal. The waste of current is due to the polarization of the metal below its equilibrium potential with the excess current being used to evolve hydrogen. The gas produced could cause the detachment of organic coatings and the removal of calcareous deposits in offshore structures. Hydrogen production has also adverse effects on both the corrosion fatigue life and hydrogen embrittlement properties of structures especially those made of high strength materials.
During overprotection large amounts of hydroxyl ions are also produced. On bare surfaces immersed in seawater, these could have a beneficial effect since the hydroxyl species may passivate and /or enhance the formation of calcareous deposits which in turn will reduce the current demand. However for organically coated surfaces the strong alkali condition at the metal surface may result in loss of adhesion for the paint. This phenomenon is known as cathodic disbonding.
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Introduction to Stray Current Corrosion
Stray currents which cause corrosion may originate from direct-current distribution lines, substations, or street railway systems, etc., and flow into a pipe system or other steel structure. Alternating currents very rarely cause corrosion. The corrosion resulting from stray currents (external sources) is similar to that from galvanic cells (which generate their own current) but different remedial measures may be indicated. In the electrolyte and at the metal-electrolyte interfaces, chemical and electrical reactions occur and are the same as those in the galvanic cell; specifically, the corroding metal is again considered to be the anode from which current leaves to flow to the cathode. Soil and water characteristics affect the corrosion rate in the same manner as with galvanic-type corrosion.
However, stray current strengths may be much higher than those produced by galvanic cells and, as a consequence, corrosion may be much more rapid. Another difference between galvanic-type currents and stray currents is that the latter are more likely to operate over long distances since the anode and cathode are more likely to be remotely separated from one another. Seeking the path of least resistance, the stray current from a foreign installation may travel along a pipeline causing severe corrosion where it leaves the line. Knowing when stray currents are present becomes highly important when remedial measures are undertaken since a simple sacrificial anode system is likely to be ineffectual in preventing corrosion under such circumstances.