النتائج 1 إلى 3 من 3

الموضوع: Corrosion of metals

  1. #1
    عضو ذهبي الصورة الرمزية ماجده
    تاريخ التسجيل
    Aug 2011
    الدولة
    Jordan - Zerqa
    المشاركات
    902

    افتراضي Corrosion of metals

    We've seen that the oxygen from the air can be reduced to water and that the reduction potential for this half-reaction is very positive. More positive, in fact than that for most metal reduction process. This means that any metal in contact with oxygen from the air and in the presence of an electrolyte solution will oxidize. Since mining and production of metal is a fairly energy-intensive process, it is very useful to be able to find ways to reduce or eliminate the oxidation of these metals if at all possible. There are several ways to reduce oxidation of metals
    Eliminate the oxygen/metal contact

    This is achieved by coating the metal with some material that is impervious to either the oxygen or to any aqueous salts. This prevents the electrical circuit from being complete in the 'electrochemical cell' that is spontaneously set up at the point of rusting.

    To keep the O2 or the salt solution, we can use several methods
    1)Insulate the metal
    a) Paint the metal. This is a pure insulating technique and serves merely to keep the reacting species separated. Unfortunately any small scratch will allow the reaction to start up and may even accelerate the process.
    b) Oil the metal. (various rust-proofing companies use this technique). The unfortunate problem is that oil is a volatile liquid and eventually either runs off the surface or evaporates away, leaving the metal unprotected. Special surfactants can be added to the oil to make it cling to the metal surface and slow evaporation. You will find these surfactant agents in treatments such as RustCheck and The new Canadian Tire rust-proofing treatments. No matter, these treatments must be repeated regularly (yearly) to maintain protection.
    c) anodize the metal: sometimes, the oxides of the metal are very stable and act to insulate the metal from the air. This is most obvious in copper roofing where the (orange) copper metal is quickly oxidized to the green oxide CuO. This oxide is quite stable and protects the underlying metal from further corrosion. This is useful in that small scratches quickly 'seal themselves up' with more oxide and the process is halted. Aluminum is often deliberately anodized in a controlled electrochemical cell to form an aluminum oxide coating that is relatively stable (in neutral solutions).
    2) Coat the metal with a material that oxidizes easier than the metal you wish to protect. For example, steel coated with zinc will not oxidize until first all the zinc is oxidized because the zinc will change the electrical potential of the steel and prevent it from being oxidized even if it is exposed to the air and water. This protection will last until the zinc is used up. at which time, the steel will start to oxidize. Cathodic protection

    a) Use a sacrificial anode made of a material that oxidized easier than the metal you wish to protect. This is similar to coating the metal with the material (as in galvanized steel) but is simpler to do since it merely means attaching chunks of the anode material rather than going through a coating process.
    Pipelines are often protected this way, the steel pipes are attached by a wire to a large block of zinc buried next to the pipeline in the ground. The zinc serves to electrically charge the steel changing its potential and thus reducing the amount of oxidation. b) Electrically change the metal's potential. Similar to the use of sacrificial anodes, this process involves attaching the negative pole of an electrical system to the metal we need to protect. In cars, for example, the negative pole of the battery is the ground. This serves to charge up the car and change it's potential. Thus, the oxygen has to overcome both the iron's reduction potential as well as the batteries artificial potential to force the reaction to happen. Cars would rust out far quicker it they were grounded positive since that, in effect, would serve to increase the rate of oxidation of the steel.

    عضو في نادي ماركا الأكاديمي


  2. #2
    عضو ذهبي الصورة الرمزية م.عبد الرحمن
    تاريخ التسجيل
    Aug 2011
    المشاركات
    261

    افتراضي رد: Corrosion of metals




    عضو في نادي ماركا الأكاديمي


  3. #3
    عضو مميز الصورة الرمزية قمر بلحاج
    تاريخ التسجيل
    Jul 2012
    المشاركات
    5,302

    افتراضي رد: Corrosion of metals

    جزاكم الله خيرا

    عضو في نادي ماركا الأكاديمي


معلومات الموضوع

الأعضاء الذين يشاهدون هذا الموضوع

الذين يشاهدون الموضوع الآن: 1 (0 من الأعضاء و 1 زائر)

الكلمات الدلالية لهذا الموضوع

المفضلات

ضوابط المشاركة

  • لا تستطيع إضافة مواضيع جديدة
  • لا تستطيع الرد على المواضيع
  • لا تستطيع إرفاق ملفات
  • لا تستطيع تعديل مشاركاتك
  •