CORROSION“ A Simple view on a complex matter”

• Virtually no practical and engineering material is stable. The rate of decay varies depending upon the material and its environment.

• The human system cannot endure – Temp above 50C or below 20C – Pressures above or below our atmospheres – Presence of harmful gases such as CO, H2S,

and H2.

• Thus, analogous to human system, engineering materials also require protection like– coatings, inhibitors, alloy addition, design

procedures, maintenance, inspection and re-furnishing.

Corrosion is the disintegration of an engineered material into its constituent atom due to chemical reactions with its surroundings. In the most common use of the word, this means electrochemical oxidation of metals in reaction with an oxidant such as O2.

EFFECTS OF CORROSION• Reduces Strength

• Life time is reduced

• Metallic properties are lost

• Wastage of metal

IMPORTANCE OF CORROSION DATA

5 mpy Good corrosion resistant material

5 to 50 mpy Low corrosion resistant material

50 mpy Unsuitable as constructional material

EXAMPLES OF CORROSION

MICROBAL CORROSIO

N

GALVANIZED SURFACE

GOLD NUGGETS

RUST

CHEMICAL EQUATION OF CORROSION

Fe2O3.XH2

O

TYPES OF CORROSION UNIFORM GALVANIC

PITTING

STRESS

MICROBIAL

EROSION

UNIFORM CORROSION CAN BE A GOOD OR A BAD THING.

CORROSION OCCURS EVENLY OVER THE SURFACE. OXIDE LAYER CAN BE VERY TOUGH – MAGNETITE.

FE3O4

GALVANIC CORROSION o CHEMICAL

REACTIONS.

o ELECTRONS REMOVED FROM ONE REACTANT TRAVEL THROUGH AN EXTERNAL CIRCUIT.

oMETERIAL TENDS TO DISAPPEAR.

PITTING CORROSION

STRESS CORROSIONBRITTLE CRACKS FORM AT THE SITES OF STRESS.

FAILURE CAN BE FAST.

FAILURE CAN OCCUR AT STRESS LOADS FAR BELOW YIELD STRENGTH.

THREE CONDITIONS REQUIRED FOR MOST COMMON KIND ARE -----

METAL UNDER TENSILE STRESS

DISSOLVED OXYGEN

CHLORIDE ION

ERROSION CORROSION• FLOW REMOVES PROTECTIVE

LAYER.

• NEW POTECTIVE LAYER FORMS USING UP METAL.

MICROBIAL CORROSION

SIMILAR TO PITTING CORROSION.

BACTERIA IN WATER.

PREVENTIONS OF CORROSION

ACIVE CORROSION PROTECTION

PERMANENT CORROSION PROTECTION

ACIVE 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.

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 & chemical factors are relatively slight in this situation.

• For example, in factories shed are protected from extreme variations in temperature, which is frequently the cause of condensation.

Created & Edited by –

Abhi Gambhir (10th A)