Pitting freshwater

Unlike the freshwater biota, POPs in local marine fish and shellfish were relatively well studied. Data were retrieved primarily from two toxic substances consultancy studies (EPD, 2003a,b), the EPD ad hoc baseline survey (EPD, 2003c), and the 2004 CEDD Environmental Monitoring and Audit for Contaminated Mud Pit IV at East Sha Chau (CEDD,... 

Austenitic SS s are also used in freshwater. However, because of cost their use is limited mainly to applications in which copper-zinc alloys are unsuitable, as in tubes in which the process side is incompatible with copper base alloys. To avoid pitting, type 304 (UNS S30400) SS is normally limited to services in which the chloride ion concentration is at a maximum of 100 ppm, and type 316 SS is limited to services in which the chloride ion is a maximum of 500 ppm. The relative pitting and crevice corrosion resistance of SS alloys can be approximated by the following equation ... 

Acid solutions, excluding hydrochloric e.g., phosphoric, sulhiric, most conditions, many organics Neutral solutions, e.g., many non oxidizing salt solutions, chlorides, sulfates Caustic and mild alkalies, excluding ammonium hydroxide Neutral or alkaline solutions, e.g, persulfates, peroxides, chromates Pitting media, acid ferric chloride solutions Freshwater supplies Seawater Steam Furnace gases with incidental sulfur content ... 

The biggest corrosion concern in freshwaters is perforation by pitting. Therefore, one wants to experimentally establish the maximum depth of pitting, usually as a function of time, so that a corrosion rate can be... 

Early on, Goddard showed in several himdred tests that pitting of aluminum in freshwater followed a cube-root curve d, = where d, is the maximum pit depth at time f/. The time to penetration can then be calculated by the formula di). The significant conclusion from the cube-root... 

Seawater tends to cause somewhat deeper pitting than does freshwater. However, early tests [8,14,15, and others] have established that the rate of pitting again follows a cube-root curve and becomes self-limiting with time. 

There are large numbers of reported case histories of MIC on stainless steel in water and aqueous waste systems. They are related to different industrial applications such as freshwater storage and circulation systems in nuclear power plants [103, 113,116,142] and cooling water systems in chemical process industries [117,118]. There are basically three cases (a) crevice corrosion under unexpected deposits, (b) sensitivity of pitting and crevice corrosion to trace of H2S, and (c) crevice corrosion in natural seawater. Most of these reports are not well documented concerning the microorganisms involved in the process. However, some general features are... 

Experience has shown that in most cases, the rate of deepening of pits formed in natural environments such as freshwater, seawater, and rain water decreases with time. This explains the very long lifetime (several decades) of aluminium used in construction (roof sheet), in naval construction, etc. 

However, there is yet another reason the corrosion resistance of aluminium in water is a topic that poses many problems for corrosion experts [2]. The great variety in the chemical composition of waters (there is no such thing as two identical waters) and the risk of pitting corrosion in contact with freshwater, especially stagnant water, makes it difficult to predict the corrosion resistance in a given water. Consequently, there has been no development of applications for aluminium in water distribution networks. 

As in the case of atmospheric corrosion (see Section C.5.2), experience shows that in most cases, the penetration rate of pitting in contact with waters (freshwater, seawater) decreases with time. Trials conducted in the early 1950s with 25 different Canadian waters [4] showed that the deepening rate of pitting on alloy 1100 follows the equation ... 

Experience shows that in certain cases, pitting corrosion in contact with water may develop during the first weeks of service. For example, in freshwater tanks, significant pitting corrosion can be observed after a few months of service as large pits, several millimetres in diameter and 1-2 mm in depth, covered by white pustules of alumina gel. 

In freshwater up to 60-70 °C, the dominating corrosion tendency is pitting. At higher temperatures, the pitting depth sharply decreases, while the density may increase in certain cases (Figure D.1.11). Above 70 °C and up to 150 °C, the propensity of pitting corrosion progressively disappears. 

C Pitting corrosion (in freshwater, the tendency to pitting corrosion decreases above 60-70 °C)... 

As in freshwater, stagnation of seawater has an unfavourable effect on the corrosion resistance of aluminium. Agitation of seawater tends to improve the corrosion resistance and leads to a decrease in pitting depth. The experience with heat exchangers of seawater... 

The resistivity of freshwater is between 1000 and 3000 fi cm , depending on its salinity. The resistivity of seawater is only lOft cm . Seawater, therefore, facilitates ionic conductivity. However, for alloys commonly used for marine applications, the two most common forms of corrosion are pitting corrosion and galvanic corrosion. 

If fuel contains traces of water of whatever origin, such as freshwater or seawater, this water may accumulate at the bottom of the tank, due to its higher density. This may lead to pitting corrosion. 

Water environments can also have a variety of compositions and corrosion characteristics. Freshwater normally contains dissolved oxygen as well as minerals, several of which account for hardness. Seawater contains approximately 3.5% salt (predominantly sodium chloride), as well as some minerals and organic matter. Seawater is generally more corrosive than freshwater, frequently producing pitting and crevice corrosion. Cast iron, steel, aluminum, copper, brass, and some stainless steels are generally suitable for freshwater use, whereas titanium, brass, some bronzes, copper-nickel alloys, and nickel-chromium-molybdenum alloys are highly corrosion resistant in seawater. 

---