mists) are projected into the air by wind and wave action. These fine 

 droplets may remain as such for some time, or the water may evaporate, 

 leaving a tiny, solid particle of salt. Wind may carry the droplets or the 

 salt particles some distance from the point of origin. It will be seen, 

 therefore, that the term marine atmosphere is not a precisely definable 

 exposure condition. The term might be applied to any situation where the 

 salt content of the air is great enough to exercise some effect on corro- 

 sivity and on protective coating performance. The produced effects may 

 range from very intense to near zero. The concentration of airborne salt, 

 both close to the shoreline and at increasing distances from it, is difficult 

 to even generally predict, since shoreline topography, wave heights, pre- 

 vailing wind direction and velocity, and inland physical features are all 

 important factors. However, the intensity of the corrosive effect declines 

 rapidly as the distance from the shore is increased and in most cases, 

 supposed acceleration of corrosion many miles inland is largely imaginary. 

 It has been reported that the effect of marine spray is negligible at 

 distances 3 kilometers (2 miles) inland and that analysis of iron corrosion 

 products at seaside towns usually shows more sulfur (from industrial 

 contamination) than chloride (from salt spray) . 



There is no doubt, however, that steel surfaces subject to atmospheric 

 exposures that are intensely marine in character present protection problems 

 which are not solved by surface preparation and paint coatings customarily 

 used for inland structures. The effects introduced by the salt are manifold 

 and varied. An obvious effect is that the corrosion-accelerating influence 

 of the salt causes even the smallest discontinuity and thin spot in the 

 coating to become a focal point for rusting which rapidly enlarges the 

 original corrosion site. This effect is heightened by the fact that the 

 corrosion products (rust) formed in salt-bearing atmospheres do not exert a 

 protective influence against further corrosion to the degree that they do 

 in inland locations; i.e., corrosion continues at a high level in marine 

 atmospheres, whereas the rate usually drops off considerably in most 

 inland atmospheres. The electrochemical reactions involved in the salt- 

 accelerated corrosion processes result in alkalies and other products which 

 may be both harmful to the paint film itself and to adhesion of the coating 

 to the metal . 



The net effect of the presence of even small amounts of deposited salt 

 is to increase the need for more care in surface preparation and paint 

 application; in more severe cases, it brings about a need for a more re- 

 sistant coating system than is customarily used on inland, weather-exposed 

 steel. The need for thoroughness in surface preparation and paint applica- 

 tion cannot be overemphasized. This need is increased by the fact that 

 crevices, joints, junctions of joining members, interior angles, pockets, 

 undersides of horizontal and inclined members, and similar surfaces tending 

 to be protected from the direct action of rain which would wash away the 

 salt, are the places of greatest corrosion and are also the places which 

 tend to receive the poorest paint job. 



Structures exposed to moderate and moderately severe marine atmospheres 

 should receive a more advanced paint system. Thorough inspection is 

 probably at least as important as the proper choice of coating. 



6. Uses in Coastal Structures . 



Generally the generic coating systems discussed in this section have 

 found satisfactory use in the exposures shown in Table 58. 



372 



