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  1. [1]
    ابن المبارك
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    تاريخ التسجيل: Feb 2006
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    التآكل

    السلام عليكم

    أرجو مساعدتي ان امكن بخصوص corrosion الخاص بالزنك

    أعمل على project للدراسة وأحتاج معلومات عن التالي
    1. أنواع السبائك مع معدن الزنك
    2. أنواع التآكل التي تصيب هذه المعادن خاصة
    3. كيفية التحكم فيها

    أرجو من المهندسين الفاضل مساعدتي بأي أبحاث مفيدة في هذه المواضيع خصوصا باللغة الانجليزية

    شاكرا ً حسن تعاونكم في المنتدى

    من مواضيع ابن المبارك :


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  2. [2]
    chopin
    chopin غير متواجد حالياً
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    تاريخ التسجيل: Jan 2003
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    1.
    The most widely used alloy of zinc is brass, in which copper is alloyed with anywhere from 9% to 45% zinc, depending upon the type of brass, along with much smaller amounts of lead and tin. Alloys of 85–88% zinc, 4–10% copper, and 2–8% aluminum find limited use in certain types of machine bearings. Alloys of primarily zinc with small amounts of copper, aluminum, and magnesium are useful in die-casting. Similar alloys with the addition of a small amount of lead can be cold-rolled into sheets. An alloy of 96% zinc and 4% aluminum is used to make stamping dies for low production run applications where ferrous metal dies would be too expensive
    2
    Corrosion is deterioration of essential properties in a material due to reactions with its environment. It is the loss of an electron of metals reacting with water or oxygen. Weakening of iron due to oxidation of the iron atoms is a well-known example of electrochemistry (a branch of chemistry that studies the reactions that take place when an ionic and electronic conductor interfere) corrosion. This is commonly known as rust. This type of damage usually affects metallic materials, and typically produces oxide(s) and/or salt(s) of the original metal. Corrosion also includes the dissolution of ceramic materials and can refer to discoloration and weakening of polymers by the sun's ultraviolet light.

    Most structural alloys corrode merely from exposure to moisture in the air, but the process can be strongly affected by exposure to certain substances .Corrosion can be concentrated locally to form a pit or crack, or it can extend across a wide area to produce general deterioration.
    3
    While some efforts to reduce corrosion merely redirect the damage into less visible, less predictable forms,
    controlled corrosion treatments such as passivation and chromate-conversion will increase a material's corrosion resistance.

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    Don’t Stop

  3. [3]
    ابن المبارك
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    جديد


    تاريخ التسجيل: Feb 2006
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    wITH ALL Thanks

    MY Dear friend chopin


    Thank you so much for ur help

    if u have any project or serach can give me more information about

    same idea please send it to me

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  4. [4]
    جيهان كمال
    جيهان كمال غير متواجد حالياً
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    تاريخ التسجيل: Sep 2005
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    Corrosion is the destructive attack of a material by reaction with its
    environment. The serious consequences of the corrosion process have
    become a problem of worldwide significance. In addition to our everyday
    encounters with this form of degradation, corrosion causes plant
    shutdowns, waste of valuable resources, loss or contamination of product,
    reduction in efficiency, costly maintenance, and expensive overdesign;
    it also jeopardizes safety and inhibits technological progress.
    The multidisciplinary aspect of corrosion problems combined with the
    distributed responsibilities associated with such problems only
    increase the complexity of the subject. Corrosion control is achieved by
    recognizing and understanding corrosion mechanisms, by using corrosion-
    resistant materials and designs, and by using protective systems,
    devices, and treatments. Major corporations, industries, and government
    agencies have established groups and committees to look after
    corrosion-related issues, but in many cases the responsibilities are
    spread between the manufacturers or producers of systems and their
    users. Such a situation can easily breed negligence and be quite costly
    in terms of dollars and human lives.
    2. BACKGROUND
    Unfortunately, in almost all instances, the cladding once installed, even if initially watertight with the
    correct seals and mastic, can be damaged by mishandling, e.g. walking across the pipes, damaging the
    protective cladding and thus allowing water ingress to occur. Although this may not appear
    immediately serious on onshore sites, especially in dry areas, if there is any water present ideal
    corrosion conditions do exist, pitting can occur and, consequently, premature failure of the pipe.
    Offshore the situation is obviously much worse, where sea water can be used for hosing down and also
    possibly as part of a fire control deluge system which is often regularly tested.
    As well as these insulated hot areas, on most chemical and petrochemical plants there are also high
    temperature areas with no insulation, such as flare stacks and exhausts. All the time these are
    operating at high temperature there is very little corrosion problem, but if there is cycling between
    high temperature and ambient then corrosion does become possible once the temperature drops below
    100ºC (212ºF). These areas are normally coated with a high temperature paint system which tends to
    remain intact all of the time the structure operates at high temperature, but can crack and flake when
    subjected to temperature cycling and thus ceases to give protection at the lower temperatures where it
    is needed.
    Traditionally, pipework on a process plant was painted in situ with oleoresinous type coatings and
    then lagged, however, this does not really fit with modern construction methods, and the advent of
    zinc silicate coatings with their inorganic characteristics with regards temperature resistance, excellent
    corrosion resistance, and resistance to mechanical damage during handling, seemed to be the solution
    to protection in all high temperature areas.
    However, in the wet situations under insulation previously described, failures started to be observed
    sometimes leading to actual perforation of the pipe. There are various theories to why this severe
    corrosion occurs:-
    · Polarity reversal in sodium chloride solution at 70-80ºC (158-176ºF), so that the steel becomes
    anodic and protects the zinc. This is recorded in the literature for zinc metal but experimental data
    on this phenomena for zinc silicate is difficult to find and we have not been able to reproduce the
    severe pitting in the laboratory, although slight pitting corrosion has been induced with cyclic
    wetness and temperature.
    · The zinc is simply more soluble in the warm water present which can be slightly acidic or alkaline
    (this can increase due to evaporation), and due to the amphoteric nature of the zinc any move from
    neutral pH will cause an increase in solubility.
    Due to these problems a divergence of approaches of corrosion engineers has occurred. Many follow
    the NACE recommendation that zinc silicate should not be used under insulation, in any form; even if
    topcoated, and others take the view that the benefits during construction of using zinc silicate
    outweigh the potential problems and that, in any case, the corrosion problem can potentially be
    alleviated by sealing off the zinc silicate with a suitable primer and thus obtaining all the benefits.
    3. THERMAL INSULATION
    Clearly then, for most circumstances, corrosion under insulation can be prevented in two ways :-
    · By using a coating system which will prevent corrosion in the potentially hot, wet conditions
    existing under the insulation.
    · Design insulation which will not be easily damaged and will prevent water ingress, either by
    nature of the insulation or by an alternative more effective method of cladding.
    Current insulation is difficult and time consuming to install and is very labour intensive, and typically
    will have to be replaced 2-3 times during the lifetime of a plant at high cost and disruption. An
    alternative method of insulation is therefore proposed which, although initially more expensive, will
    not require the normal maintenance and is designed to eliminate the corrosion problems. Because the
    material can be either installed by spray direct to the substrate on large areas, or using pre-cast half
    shells on piping, sealed and glued to the surface, water ingress and hence corrosion has been
    eliminated.
    This has been achieved by utilising two phase and three phase epoxy syntactic foams. At first sight it
    may be thought that epoxy will not have sufficient temperature resistance, however, research shows
    that on most plants more than 90% of the hot steel is operating below 120ºC (248ºF), and thus this
    material can be used in the majority of circumstances.
    4. HIGH TEMPERATURE ANTI-CORROSIVE COATINGS
    Concurrently with the development of this insulation material, anti-corrosive primers suitable for
    overcoating the steel after blasting, capable of operating at the specified temperatures and allowing
    excellent adhesion of the insulation, have been formulated, based on epoxy phenolic resin systems.
    This is one possible approach to the prevention of corrosion of hot surfaces, especially those which are
    insulated, but realistically is unlikely to achieve universal acceptance in the short to medium term,
    consequently, a more conventional approach has also been considered which can obviously be utilised
    on uninsulated steel, as well as insulated surfaces.
    As mentioned previously, there have been many problems with zinc silicate based systems when used
    under insulation which can potentially become wet, and there have been other and different problems
    with high temperature areas, i.e. greater than around 200ºC (392ºF) , where protective topcoats on the
    zinc have in many cases failed through lack of adhesion or blistering.
    It is necessary to consider this high temperature atmospheric scenario further, the zinc silicate is
    basically present to give corrosion resistance during construction, and whilst the plant is not operating
    at high temperature. The aluminium silicone sealer which is normally used is present to prevent
    oxidation of the zinc particles in the zinc silicate, which occurs more quickly at elevated temperatures
    and is thought would reduce corrosion protection, destroying galvanic contact. This is correct but, in
    fact, the formation of the oxide, and other salts, gives a more effective barrier which although not
    having cathodic protective properties has superior barrier properties. There are many reported
    instances of untopcoated zinc silicate protecting for many years at temperatures above the melting
    point of zinc, presumably because of this effect. Generally the aluminium silicone is present for
    aesthetics and to protect zinc from alkaline or acid conditions.
    There is a considerable lack of understanding of the performance parameters of these aluminium
    silicone based coatings. Basically they are designed to be applied at dry film thicknesses of 15
    microns (0.6 mils), (not the often specified 25 microns (1 mils)), and require stoving at around 200ºC
    (392ºF) to give a cured film with optimum film properties. Application at dry film thicknesses of
    greater than 15 microns (0.6 mils) can lead to blistering and adhesion loss, primarily caused by the
    water vapour generated by the curing mechanism (Figure 8). It is always much safer to use one of the
    new ambient curing (moisture curing) systems which are more tolerant in film thickness, do not
    require heat curing, and allow application of multi-coat systems without heating between coats.

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  5. [5]
    جيهان كمال
    جيهان كمال غير متواجد حالياً
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    تاريخ التسجيل: Sep 2005
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    The problem with this type of material is that although it gives sufficiently thick (40-60 microns, 1.6-
    2.4 mils) films in multi coats to prevent corrosion of zinc silicate primed steel, films without this
    primer do not have good corrosion resistance, mainly due to too low a film thickness. This potentially
    causes problems on insulated high temperature steel (>250ºC, >482ºF) where because of concerns
    regarding wet conditions zinc silicate may not be used, but 60 microns (2.4 mils) of aluminium
    polysiloxane certainly does not give the required hot water resistance.
    Recent developments have focussed on a number of areas to try to improve the coatings industry
    solutions to these problem areas.
    These have concentrated on alleviating the limitations previously identified in the current portfolio of
    materials available., i.e.
    · Organic systems, e.g. epoxy phenolic, maximum operating temperature 230ºC (446ºF).
    · Zinc silicates – well documented.
    · Aluminium silicones etc – insufficient thickness to give good barrier properties.
    · High build polysiloxanes – limitations in repeated temperature cycling.
    Test methods have had to be developed to evaluate coating performance in a number of situations, i.e.
    · Cyclic insulated high temperature piping with intermittent wetting.
    · High temperature exposure followed by quenching in water.
    · High temperature exposure followed by accelerated corrosion testing or natural weathering.
    · Natural weathering followed by high temperature exposure.
    · Accelerated testing followed by high temperature exposure.
    Examples of developmental results are shown in Figures 9 to 14. Excellent corrosion resistance has
    been obtained after heating, and although accelerated corrosion performance of ambient cured systems
    may give poor performance, in a C5M environment appears satisfactory for 6 months to date. No
    defects are seen after subsequent high temperature cycling.
    Thus to summarise, a system has been developed to replace conventional insulation at temperatures up
    to 150ºC (302ºF), and significant progress has been made in the development of a “universal” high
    temperature anti-corrosive system for temperatures up to 400ºC (752ºF) and which is zinc free.

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  6. [6]
    abu- kamhawy
    abu- kamhawy غير متواجد حالياً
    جديد


    تاريخ التسجيل: May 2009
    المشاركات: 1
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    للاهميه القصوى
    مشكور اريد ملف iso 4628

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