وزارة التعليم العالي والبحث العلمي

4th Class By

Lecturer: Basheer Ahmed

2015-2016

Corrosion Engineering

Still other consequences are social. These can involve the following issues: _ Safety, for example, sudden failure can cause fire, explosion, release of toxic product, and construction collapse _ Health, for example, pollution due to escaping product from corroded equipment or due to a corrosion product itself _ Depletion of natural resources, including metals and the fuels used to manufacture them _ Appearance as when corroded material is unpleasing to the eye

1. General Corrosion, or Uniform Attack. This type of corrosion includes the commonly recognized rusting of iron or tarnishing of silver. “ Fogging ” of nickel and high - temperature oxidation of metals are also examples of this type.

Rates of uniform attack are reported in various units, with accepted terminologies being millimeters penetration per year (mm/y) and grams per square meter per day (gmd). Other units that are frequently used include inches penetration per year (ipy), mils (1 mil = 0.001 inch) per year (mpy), and milligrams per square decimeter per day (mdd).

2. Pitting. This is a localized type of attack, with the rate of corrosion being greater at some areas

than at others. If appreciable attack is confined to a relatively small, fixed area of metal, acting as anode, the resultant pits are described as deep. If the area of attack is relatively larger and not so deep, the pits are called shallow. Depth of pitting is sometimes expressed by the pitting factor, the ratio of deepest metal penetration to average metal penetration as determined by the weight loss of the specimen. A pitting factor of unity represents uniform attack (Fig. 2.7 ).

3. Intergranular Corrosion. This is a localized type of attack at the grain boundaries of a metal,

resulting in loss of strength and ductility. Grain – boundary material of limited area, acting as anode, is in contact with large areas of grain acting as cathode. The attack is often rapid, penetrating deeply into the metal and sometimes causing catastrophic failures. Improperly heat - treated 18 - 8 stainless steels or Duralumin - type alloys (4% Cu – Al) are among the alloys subject to intergranular corrosion. At elevated temperatures, intergranular corrosion can occur because, under some conditions, phases of low melting point form and penetrate along grain boundaries; for example, when nickel - base alloys are exposed to sulfur - bearing gaseous environments, nickel sulfide de can form and cause catastrophic failures

4. Cracking. If a metal cracks when subjected to repeated or alternate tensile stresses in a

corrosive environment, it is said to fail by corrosion fatigue . In the absence of a corrosive environment, the metal stressed similarly, but at values below a critical stress, called the fatigue limit or endurance limit , will not fail by fatigue even after a very large, or infi nite, number of cycles. A true endurance limit does not commonly exist in a corrosive environment: The metal fails after a prescribed number of stress cycles no matter how low the stress. The types of environment causing corrosion fatigue are many and are not specific

2.2 DEFINITION OF ANODE AND CATHODE A combination of two electrical conductors (electrodes) immersed in an electrolyte is called a galvanic cell in honor of Luigi Galvani, a physician in Bologna, Italy, who published his studies of electrochemical action in 1791. A galvanic cell converts chemical energy into electrical energy. The electrode at which chemical reduction occurs (or + current enters the electrode from the electrolyte) is called the cathode . Examples of cathodic reactions are

all of which represent reduction in the chemical sense. The electrode at which chemical oxidation occurs (or + electricity leaves the electrode and enters the electrolyte) is called the anode. Examples of anodic reactions are:

When current is impressed on a cell from a generator or an external battery — for example, as in electroplating — reduction occurs at the electrode connected to the negative pole of the external current source, and this electrode, consequently, is the cathode. Similarly, the electrode connected to the positive pole of the generator is the anode. It is perhaps best, therefore, not to

Cations are ions that migrate toward the cathode when electricity flows through the cell (e.g., H + , Fe 2+ ) and are always positively charged whether current is drawn from or supplied to the cell. Similarly, anions are always negatively Charged (e.g., Cl − , OH − , SO4 2− ).