9.S.S POLLUTION Atmospheric contaminants, mainly salts and SO2, in combination with humidity contribute to the corrosion of most metals. Chloride ions breakdown the passive layer of oxides on steels and initiate pitting. Compounds of nitrogen, such as ammonia, cause serious problem on copper and brass structures by formation of Cu(NH3) complexes. Copper piping networks can be seriously damaged by corrosion in agricultural areas where ammonia may be present. Aluminum alloys are sensitive to corrosion in an environment with a pH range less than 4 and greater than 10, the former representing the acid regimes and the later the alkaline regimes corrosive to aluminum.

9 . 6 RANKING OF PERFORMANCE OF MATERIALS The ranking of materials is based on the basis of the degree of corrosion resistance they offer in a given environment. The best way to evaluate corrosion resistance is to determine their rate of corrosion according to ASTM (American Society for Testing of Materials) and NACE (National Association of Corrosion Engineers) standards. Important tests to determine the rate of corrosion are given below: (1*) ASTM Designation G5. Annual Book of Standards, Vol. 03, 02. 2001 (2*) ASTM Designation G31. Annual Book of Standards, Vol. 03, 02. (American Society for Testing of Materials, 2001) (3*) ANSI/ASTM Designation G51. Annual Book of Standards, Vol. 02, 03. Conducting Cyclic potentiodynamic Polarization Measurement for Localized Corrosion. (American Society for Testing of Materials, 2002)

CORROSION BEHAVIOR OF TITANIUM IN SPECIFIC ENVIRONMENT Service experience stretching over three decades in field studies and labs around the world has proved that titanium offers an excellent resistance to corrosion in polluted and clean seawater up to 130°C. It also shows an excellent resistance to corrosion in distilled water and tap water. It resists temperatures up to 400° C in an autoclave. It is believed that no other metallic material is more resistant to seawater corrosion than titanium. The corrosion resistance of Ti and its alloys in to HC1, H2S04, HNO3 and H3PO4 is given below. (a) Hydrochloric acid Titanium can be safely used in 3% HC1 at 60°C and 0.5% HC1 at 100°C. It is only moderately resistant due to the non-oxidizing nature of HC1. Corrosion of titanium in HC1 is given in Fig. 9.20. (b) Sulfuric acid The corrosion rate of titanium in H2SO4 follows a linear relationship up to 20% concentration (0.5 mm/year), followed by a peak

. Presence of 0.25% CUSO4 reduces the corrosion rates of titanium in 30% H2SO4 from higher rates to lower rates, such as 0.06 mm/year. (c) Phosphoric acid The corrosion behavior is similar to that in HC1. The corrosion rates vary from being low (0.003 mm/year in 5% H3PO4 at 35°C) to high corrosion rates (17.4 mm/year in 30% H3PO4 at 100°C). (d) Nitric acid Due to the oxidizing nature of HNO3, Ti exhibits low corrosion rates in HNO3. Titanium is strongly resistant to attack by a mixture of concentrated H2SO4/HNO3.

CORROSION PREVENTION BY DESIGN The important point is that the designers must have an understanding and awareness of corrosion problems. Corrosion is, however, only one of the several parameters with which the designer is concerned and it may not be, however, important to a designer to give consideration to corrosion unless dictated by a requirement. SERVICE LIFE OF EQUIPMENT Selection of a corrosion resistant material for the environment is a prerequisite to a good design. Materials and design are complimentary to each other and neither of the two can be ignored. The following factors influence the service life of equipment (Fig. 8.1): 1) Environments and geographic location 2) Selection of materials 3) Maintenance 4) Corrosive environment and velocity of flow 5) Design 6) Feature promoting corrosion 7) Bimetallic connection.

8.3 CAUSES OF FAILURES IN THE CONTEXT OF DESIGN A good engineering design should provide a maintenance-free service, satisfy the end user, and provide a maximum return on capital in a shortest return period. CORROSIVE ENVIRONMENT The following are the major ingredients of an atmospheric corrosive environment: (1) Temperature (2) Humidity (3) Rainfall (4) Air pollution (5) Proximity to sea (6) Dust storms and dust particles.

Following is a summary of the effect of major contributors to corrosive environments. Temperature Humidity Rainfall Pollution Man-Made Pollution Proximity to Sea STAGES IN THE DESIGN PROCESS The following are the four important stages in the design process: 1) Clarifying objectives - To clarify design objectives 2) Establishing functions - To define the function requirements 3) Setting requirements - To specify the performance 4) Improving details - To increase the value of a product and making it cost effective

CONSIDERATION OF AREAS REQUIRING ATTENTION AT DESIGN STAGE The following are the areas which require attention to minimize corrosion: • Bimetallic contacts • Faying surface • Crevices • Moisture traps • Water traps • Metals in contact with moisture absorbent materials * Inaccessibility • Areas of condensation • Features which reduce the paint thickness • Welds • Oil, grease and rust patches • Fluid movements • Joints (threaded, riveted and screwed) • Closed sections and entrapment areas • Mechanical factors • Corrosion awareness

DESIGNS FOR LIQUID CONTAINERS A good design for liquid container must offer the following: a) Freedom from sharp corners and edges. b) Smooth flow of liquid from the container. c) Freedom from the buildup of water traps around the corners. d) Complete drainage from the corners without any water traps. The elimination of water traps is essential to minimize the formation of differential oxygen cells which lead to corrosion. As an operational matter, it is essential to remove water and dry out stainless steel tubing without delay as soon as leak testing of new water treatment plant is completed; there are many examples of microbial corrosion causing severe pitting of new plant soon after leak testing. e) Minimizing of bimetallic corrosion by joining compatible materials without the risk of galvanic corrosion. f) Complete internal and external coating of the containers, if cost effective.