ResultExamplePreventionEnvironmental effect (Factor effect)MechanismCauseCharacteristicForm of corrosion

Uniform corrosion is usually the result of breakdown in protective coatings-A piece of steel or zinc immersed in dilute sulfuric acid will normally dissolve at a uniform rate over its entire surface .-Rust1. Proper material selection 2. Change the environment (add inhibitors) 3. Cathodic protection1. pH Low pH accelerated the cathodic reaction pH affects the stability of passive films2. Dissolved gas Dissolved oxygen3. Velocity Higher velocity increases mass transfer rate Accelerates erosion4. Temperature Increased temperature, will increase reaction rateIn uniform corrosion the electrochemical reactions occur at the same rate over the entire surface (corrosion occurs evenly over the entire surface of the corroding metal).- Uniform corrosion proceeds by a chemical / electrochemical reaction uniformly over the surface.

- This type of attack is mostly found where a metal is in contact with an acid, a humid atmosphere or a solution in general.

-The metal becomes thinner and eventually fails .A-Uniform (general)

1. Environment Oxygen concentration Chloride ion concentration pH flow rate

2. Material Inclusions Different phases Grain boundaries

3. mechanical Static stress Cyclic stressB- Localised corrosion

DrawingShape of apperanceExamplePreventionEnvironmental effect (Factor effect)MechanismCauseCharacteristicForm of corrosion

Dry Cell battery

1. Select combinations of metals as close together as possible in the galvanic series. 2. Avoid the unfavorable area effect of a small anode and large cathode. 3. If dissimilar metal must be joined, Insulate them electrically wherever practicable. 4. Apply coatings with caution. 5. Add inhibitors, if possible, to decrease the aggressiveness of the environment. 6. Design for the use of readily replaceable anodic parts or make them thicker for longer life. 7. Install a third metal which is anodic to both metals in the galvanic contact, i.e. cathodic protection1- potential reduction (E)

2- Distance Effect

3-Area effect-The more active (anodic) metal corrodes faster-The less active (anodic) metal corrodes slower and will be protected-Driving force for corrosion the difference in electrode potentialDifference in potential between metals (anode and cathode) in the galvanic seriesOccurs when 2 different metals are electrically connected in the same electrolyteB1-Galvanic or Two metal corrosion

1-Using material that their Susceptibility is less than other for pitting corrosion2- using inhibitors to decrease pitting corrosion - it is a unique type of anodic reaction - it is autocatalytic process 1- scratches or other defect such than micro flows2- random variation on solution composition as PH 3- non homogenously in oxygen concentration

Pitting corrosion is an intensive localised attack which results in the development of cavities or "pits" in the metal surfaceB2- Pitting corrosion

*Pitting can be one of the most dangerous forms of corrosion because it is difficult to anticipate and prevent, relatively difficult to detect, occurs very rapidly, and penetrates a metal without causing it to lose a significant amount of weight.

The crevice can be a hole, a space between the surface a poorly adherent coating, another metal or even a non-metal (concrete, glass, rubber).Examples of such crevices are: flanged or threaded connections, shielded areas on metal surfaces (eg as a result of deposit formation)1- The most effective way of minimizing crevice corrosion is the elimination of the crevice itself2- Crevice corrosion can be controlled by good design geometry (shape) to remove the crevice itself

** method to minimizing crevice corrosion (Textbook Page 59)Such stagnant microenvironments tend to occur in crevices (shielded areas) such as those formed under gaskets, washers, insulation material, fastener heads, surface deposits, disbonded coatings, threads, lap joints and clamps.

- Corrosion rate of crevice is higher than that on bulk-Crevice corrosion occurs mainly on alloys which passivate (e.g; stainless steels)-the same mechanism of pitting corrosion but here is Crevice not pit Liquid entry but stagnant (not flowing)Crevice corrosion occurs at shielded areas that contain small volume of aqueous solutionB3- Crevice corrosion

* Crevice corrosion is a type of pitting corrosion that occurs specifically within the low flow region of a crevice.

*Condition to occur this type of corrosion : 1- crevice should be presented as wide enough to permit to enter the solution corrosive environment and narrow to maintain the stagnate 2- existence of corrosive environment

Like a filament has a head and tail . The head is responsible of spreading a filament and has a blue green color Tail has red brown color - the cause of blue green color is Fe+2 and of red brown color of Fe+31- Maintain low humidity 2- increase the thickness of proper coating 3- brittle film of covering surface 1- Humidity : if humidity increasing more than 90% the filament become a tunnel -By existence humidity H2O and O2 are penetration under the coating layer and O2 is reduction O2+H2O+4e-4OH-- The head of filament is supplied with water from surrounding by osmotic phoneme due to the high concentration of ferrous ions (Fe+2) at the same time osmosis remove water from tail because of the lack of concentration of soluble salt. So corrosion is restricted in area of head .So the process occur is the autocatalytic phenomenon The cause of this type is differential oxygen concentration and existence of water

It is a special form of corrosion that occur under a thin coating layers in the form randomly distributed threads like filaments B4-Filiform corrosion (under film corrosion)

grooves1- lowering carbon content to below 0.03%2- Adding stabilizes material such as titanium and nibonum 3- heat treatment process such as quenching It is the corrosion which occur the grain boundary of solidified material*condition :1-Cr>10%2-C more than 0.03%3-Temp. (950-1450C)4- Tensile stress* The cause of this type is differences in composition 1- impurities on grain boundary 2- depilation of an element in grain boundary 3- enrichment of alloying element It is a cracking of material that occur along grain boundary in the presence of tensile stress - it can be seen by microscopic of magnification rate =3X B5-Intergranular corrosion (intercrystaline)

Grooves and valleys AL , Steel ,Cu , Lead1- material selection not susceptible to this kind 2- good design (shape, geometry,) 3- alteration of environment such as : a-Deaeration b- adding inhibitors c- heat treatment d- type of flow must be laminar not turbulent 4- coating 5-cathodic protection At the initial stage a mechanical removed the film that cover the metal (passive or part of metal) them the corrosion bare metal occur by flowing fluid movement in the surround 1- hot gases oxides metal in cyclic motion at high velocity 2- slurries are distractive to most metals in pumps and impellers To occur this type must be the following condition :mechanical+ chemical + moving fluidB6- Erosion corrosion

- Erosion corrosion : it is acceleration or increase in rate of deterioration because the movement between corrosive fluid and surface of metal .- the movement must be rapid motion and cyclic

(Shape like pits)1- by design to minimize hydro dynamic pressure difference 2- adding inhibitors 3- smooth finishes on pump impeller4- coating metallic parts 5- cathodic protection Differential in pressure and mechanical effect and chemical corrosionit is a special case form of Erosion corrosion caused by water bubble produced by high speed impeller and then collapse and cause sever corrosion on metal surfaceB7- Cavitation damage

Penetration or interaction hydrogen with metal Localized corrosion B8- Hydrogen damage