Uniform corrosion water layers

Time of wetness (TOW), considered as the time during which the corrosion process occurs, is an important parameter to study the atmospheric corrosion of metals. According to ISO-9223 standard, TOW is approximately the time when relative humidity exceeds 80% and temperature is higher than 0°C. No upper limit for temperature is established. In tropical climates, when temperature reaches values over 25°C, evaporation of water plays an important role and the possibility to establish an upper limit respecting temperature should be analyzed. The concept of TOW assumes the presence on the metallic surface of a water layer however, there are recent reports about the formation of water microdrops during the initial periods of atmospheric corrosion, showing that the idea of the presence of thin uniform water layers is not completely in agreement with the real situation in some cases (particularly indoor exposures). 

Recent reports about the microdroplets formation in the starting periods of atmospheric corrosion [15-18] show that the idea of a thin uniform water layers is not completely in accordance with the reality. It has been observed that when a water drop is on the metallic surface, formed in the place where a salt deposit existed before, microdroplets are formed around this central drop. The cathodic process takes place in these surrounding microdroplets, meanwhile the anodic process takes place in the central drop. This idea is not consistent with the proposal of an uniform water layer on the surface and it is very probable that this situation could be obtained under indoor conditions. It has been determined that microdrops (about 1 micron diameter) clusters are formed around a central drop. An important influence of air relative humidity is reported on microdrops formation. There is a critical value of relative humidity for the formation of microdroplets. Under this value no microdroplets are formed. This value could be considered as the critical relative humidity. This situation is very similar to the process of indoor atmospheric corrosion presence of humid air, deposition of hygroscopic contaminants in the surface, formation of microdrops. Water is necessary for corrosion reaction to occur, but the reaction rate depends on the deposition rate and nature of contaminants. 

The concept of TOW assumes the presence on the metallic surface of a water layer however, there are recent reports about the formation of water microdrops during the initial periods of atmospheric corrosion, showing that the idea of the presence of thin uniform water layers is not completely in agreement with the real situation in some cases (particularly indoor exposures). 

The droplet cell. Fig. 2(d), has uniform current distribution and shrunken dimensions that allow resistive electrolytes to be used [5]. This approach was developed for the use of pure water as an electrolyte as a means to mimic atmospheric corrosion, but it can be used with any electrolyte. An area of a flat sample is exposed through a hole in a piece of protective tape. Electroplater s tape is a very resistant tape with good adhesion that is useful for this and other masking applications in corrosion. If the hole in the tape is made with a round punch, the same punch can be used to make circular dots from pieces of filter paper. One such dot is placed securely into the exposed hole. A small (typically 10-20 gl) droplet of soluhon is placed on the filter paper using a calibrated pipette. This wet filter paper acts as the electrolyte. A piece of woven Pt mesh is placed on top of the wet filter paper, and a reference electrode is held against the back of the Pt counterelectrode. As mentioned, the small dimensions allow the use of even very pure water. This simulates atmospheric corrosion, in which a thin water layer forms on the surface. As in atmospheric corrosion, soluble species on the sample surface and pollutant gases in the air are dissolved into the water droplet, which provides some conductivity. This technique has been used... 

The term protective layer should not be used in relation to the formation of hardness layers and the uniform corrosion of unalloyed steel. Newer investigations of drinking water applications (Sontheimer, 1988) indicate there is no direct correlation between the Saturation Index and the corrosion rate of unalloyed steel. Fig. 1-31, although a scaling water is generally less corrosive than a non-scaling water. 

Metal loss in these areas had produced a smooth surface, free of deposits and corrosion products. The rest of the internal surface was covered by a thin, uniform layer of soft, black corrosion product. The graphitically corroded surfaces of the pump casing provided soft, friable corrosion products that were relatively easily dislodged by the abrasive effects of high-velocity or turbulent water (erosion-corrosion). 

A practical example of almost uniform surface corrosion is as follows A pipe made of unalloyed steel St 35 used at approximately 90°C for the transport of service water showed material erosion of the inner surface after 3 years in operation. A layer of corrosion product (mainly iron oxide) had formed on the inside with a practically constant thickness over the entire area. Given the operating conditions, the material corrosion had to be due to oxygen corrosion. 

In stable passive metals, for instance stainless steels, the weak oxidant water is sufficient to effect the transition to the passive range. The presence of oxygen in the water is not required for this purpose. The passivating oxide layer is quickly replaced following a mechanical rupture (repassivation). As a rule, the balance potential of the existing redox system, i.e. the redox potential of the corrosive agent, is established within a brief period. Stainless steels are therefore preferred for use in mediums the redox potential of which passivates them. In these mediums, the uniform surface corrosion levels are so small with free corrosion that the structural element can be expected to have a technically acceptable service life. 

The corrosion mass loss of the steels in Table 18 after exposure in the North Sea water off Helgoland in the splash, tidal, and immersion zones is shown in Figure 2. The scatter bands of the seven steels are shown. As was expected, the weather-resistant steel (O) with raised levels of copper, nickel, chromium and phosphoms, which favours the formation of denser mst layer, shows the lowest mst levels in the splash zone. A steel (A) with raised silicon and manganese contents shows nearly equivalent behaviour. The corrosion rates of both steels are at the lower limit of the scatter band. In the tidal and immersion zone, on the other hand, all steels fill out the scatter band uniformly. The resulting corrosion rates are hsted in Table 19. 

Cast iron, ductile iron, or steel pipe may be coated at the mill by a process during which the pipe is spun on the center of its longitudinal axis while a mortar mixture is sprayed onto the inside surface in a uniform, dense layer. After proper curing, provided the pipe is handled carefully, this coating can protect the pipe interior against attack by water and many other liquid and gaseous corrosive environments. 

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