1. Introduction
This was an individual report requested by Professor. Shi, the instructor of the Subject: CORROSION CONTROL ME571. This report deals with several real life examples of corrosion / oxidation and will explain why and how the corrosion of metals happened or worked. The main aim of the report is to catch the cause-and-effect relationship of the corrosion and to recommend ways of preventing and controlling corrosion.
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2. Show & Tell of Atmospheric Corrosion
Atmospheric corrosion is surely the most visible of all corrosion processes, e.g. rusty bridges, flag poles, buildings and outdoor monuments. It has been reported to account for more failures in terms of cost and tonnage than any other type of material degradation processes. While atmospheres can be classified into four basic types, (1) Industrial, (2) Marine (3) Rural and (4) Indoor Atmospheric Corrosion.
The following findings can be found everywhere in real life, some or all of them are likely to live with our living areas.
3. Findings:
-Real Life Example: (Description of the atmospheric corrosion of sewage pipes)
Location: Kai Tak Nullah
Figure 1: Location of the affected sewage pipes in Kai Tak Nullah
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The pictures below (see Picture 2 and Picture 3) show the allegro wastewater was continuously splashing on the pipe surfaces, particular underneath the pipelines, such environment typically have high percent relative humidity (% ＲＨ) and will provide an aggressive electrolyte for the promotion of corrosion.
In fact, these pipelines in Kai Tak Nullah almost have 40 to 50 year history, they are in summer time exposed to a high-temperature atmosphere and their bottom sides are all the time contacting with the sewage. It can be referred-to in note 5 of this taught subject, a fundamental requirement for atmospheric corrosion processes is the presence of a film electrolyte that can form on metallic surfaces when exposed to a critical level of humidity (see picture 3 above), especially under conditions of alternate wetting and drying.
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In addition, unexpected rapid corrosion can be seen as well from the following picture, two different levels of corrosion on pipeline surfaces are viewed, the pipelines at column 2 and 3 their surfaces absorbed moisture (the pipe surfaces appeared saturated condition compared to that at column 1), and grew a thick layer of corrosive product on their surfaces.
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4. Factors affecting atmospheric corrosion
Temperature
Temperature plays an important role in atmospheric corrosion in two ways. First, there is the normal increase in corrosion activity which can theoretically double for each ten degree increase in temperature. Secondary, a little-recognized effect is the temperature lag of metallic objects, due to their heat capacity, behind changes in the ambient temperature. As the ambient temperature drops during the evening, metallic surfaces tend to remain warmer than the humid air surrounding them and do not begin to collect condensation until some time after the dew point has been reached. As the temperature begins to rise in the surrounding air, the lagging temperature of the metal structures will tend to make them act as condensers, maintaining a film of moisture on their surfaces.
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Relative Humidity (RH)RH is defined as the ratio of the quantity of water vapor present in the atmosphere to the saturation quantity at a given temperature, and it is expressed as %. The critical humidity level is a variable that depends on the nature of the corroding material, the tendency of corrosion products and surface deposits to absorb moisture, and the presence of atmospheric pollutants. It has been shown in the above example 1, the critical humidity level is predicted to be 85 to 90% because the bottom surface and the surface around the whole pipe are quite near the splash zone. In the presence of electrolytes, atmospheric corrosion proceeds by balanced anodic and cathodic reactions. The anodic oxidation reaction involves the corrosion attack of the pipe surface, while the cathodic reaction is naturally the oxygen reduction reaction.
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Wetness in A Long PeriodA resident (say me) has been living in Wong Tai Sin and living with Kai Tak Nullah for over 30 years, the mentioned pipelines installed in the said location should at least be 40 years long.
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Moisture and PollutantsDew and condensation are undesirable from a corrosion standpoint if not accomplished by frequent rain washing which dilutes or eliminates contamination. A film of dew, saturated with acid sulfates, and acid, chlorides of an industrial atmosphere provides an aggressive electrolyte for the promotion. Also, the stagnant moisture film either becomes alkaline from reaction with metal surfaces, or picks up carbon dioxide and becomes aggressive as a dilute acid.
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5. Prevention of sewage pipe corrosion is suggested in several ways:
Organic Adsorption Inhibitors, organics that coat metal with an oily surface layer will protect it. Inhibitors are commonly used in acids, although a few function in neutral or alkaline solutions. In adsorbing on the pipe surface, they replace adsorbed sewage or wetness and their hydrophobic action prevents water from solvating metal ions or prevent H+ ions from adsorbing at cathode sites where reduction to H2(g) could occur.
Plastic cover plate to be added underneath of each sewage pipe, the function of this can isolate the contact point between the splash level and the bottom of pipe
Always keeping the vertical distance between the bottom of pipe (or the bottom of the proposed plastic cover plate) and the waterline as long as possible so as to minimize the splashing effect.
Periodically check and examination of the pipeline systems are should be carried out at least 3 months in order to control and monitor easily the corrosion products.
6. Reference:Course Notes (ME571 CORROSION CONTROL)
CORROSION CONTROL Second Edition, Bradford Samuel A.
http://www.corrosion-doctors.org/Corrosion-Forms/area-effects.htm
Best Regards
Ken Ngan
