yield laterally to relieve any thrust load due to ice. Plugged and broken 

 waterlines caused by ice are inconveniences that require cooperation between 

 design and construction personnel, and between operations and maintenance 

 personnel to eliminate this prohlem. Heat tracing and insulation are solu- 

 tions to this problem, but other methods may be more practical such as 

 pumping out fire hydrants, and closing doors to heated loading docks. 



e . Exposure to Saltwater . 



(1) C orrosion Effects - General . Corrosion rates of metals in 

 seawater are higher than in pure water because ions of halogen compounds, 

 such as sodium chloride, have the power to cause localized breakdown of oxide 

 films that are responsible for passivity and corrosion resistance. Halogen 

 ions can form soluble acidic corrosion products, such as ferric chloride, 

 which interfere with the restoration of passivity to steel leading to 

 localized corrosion in the form of pitting. Tests have shown that corrosion 

 rates for carbon steel in the atmosphere at the shoreline are 10 times the 

 rates shown by plates 460 meters (.500 yards) from the shoreline. 



It has been shown that the rate of corrosion of steel in seawater and in 

 freshwater is governed to a large extent by the oxygen content. Carbon 

 steel, in contact with freshwater saturated with oxygen at ambient tempera- 

 ture, usually exhibits a corrosion rate of 220 micrometers (9 mils) per year 

 general corrosion plus an additional 220 micrometers per year of pitting. 

 When freshwater is oxygen-free, the corrosion rate for carbon steel is 

 usually only 25 micrometers (1 mil) per year or less, provided no corrosive 

 pollutants are present. 



(2) Variable Oxygen Content . The pattern of corrosion found on 

 steel pilings in the atmosphere, the splash zone, the tidal zone, submerged 

 in clean seawater, and in the mud zone varies considerably. A principal 

 variable related to position is the oxygen content. The high corrosion rate 

 in the splash zone is attributed to the constant wetting of the steel by 

 highly aerated seawater. In the tidal zone, differential aeration produces 

 a protective cell effect, resulting in a considerably lower corrosion rate. 

 At deeper positions, less oxygen is present and the corrosion rate for steel 

 drops to rates usually in the range of 76 to 152 micrometers (3 to 6 mils) 

 per year. Carbon steel in seawater that has been treated to remove dissolved 

 oxygen and marine bacteria exhibits an even lower corrosion rate under low 

 velocities . 



Austenitic stainless steel and aluminum alloys exhibit satisfactory 

 corrosion resistance in the splash zone, because the high oxygen content 

 helps keep passivating films intact. Aluminum has better corrosion resis- 

 tance in the splash zone than at greater depths where less oxygen is present. 

 The high corrosion rates on carbon steel piling in the splash zone may also 

 be attributed to the severe electrochemical corrosion cells set up in the 

 pile. Piles made from high-copper-bearing, high-strength, low-alloy steel 

 conforming to ASTM Standard A690 have two to three times the resistance to 

 seawater corrosion in the splash zone of ordinary carbon steel, although 

 such steels exhibit no better corrosion resistance at greater depths. 



(3) Effects of Polluted Seawater . Polluted waters often contain 

 hydrogen sulfide which causes severe effects in metals sensitive to the 



222 



