Fresh water, corrosion

Corrosion can also be classified based on environments. For example, we can mention the following corrosion phenomena - atmospheric corrosion, fresh water corrosion, sea water corrosion, soil corrosion, high temperature corrosion, and gaseous corrosion. 

Fresh water corrosion is the general term for all of the corrosion occurring in rivers, tap water, ground waters, etc. They usually do not contain so much chloride ion. 

Sea water corrosion can be compared with fresh water corrosion. In the former, the water contains a lot of chloride ions and it accelerates the corrosion phenomenon very much. 

Corrosion types can also be categorized based on what type of environment they take place. Accordingly, major corrosion types are atmospheric corrosion, corrosion in fresh water, corrosion in seawater, corrosion in soils, corrosion in concrete and corrosion in the petroleum industry. 

It is extensively used for making stainless steel and other corrosion-resistant alloys such as Invar(R), Monel(R), Inconel(R), and the Hastelloys(R). Tubing made of copper-nickel alloy is extensively used in making desalination plants for converting sea water into fresh water. 

Titanium has potential use in desalination plants for converting sea water into fresh water. The metal has excellent resistance to sea water and is used for propeller shafts, rigging, and other parts of ships exposed to salt water. A titanium anode coated with platinum has been used to provide cathodic protection from corrosion by salt water. 

Allowing DRI to become wet does not necessatily cause it to overheat. When large pdes of DRI are wetted with rain, the corrosion reactions are limited to the outer surface area of the pde and the resultant heat from the corrosion reactions is dissipated into the atmosphere. However, if water penetrates into the pde from the bottom, or if wet DRI is covered with dry DRI, the heat from corrosion reactions can budd up inside the pde to the point where rapid reoxidation begins. Corrosion occurs significantly faster with salt water than with fresh water. DRI saturated with water can cause steam explosions if it is batch charged into an electric arc furnace. 

Desalination. A special case of distillation is water desalination. In places where energy is abundant but fresh water is not, eg, the Arabian Peninsula, water may be produced from seawater ia flash evaporators. Low pressure turbiae steam is extracted to provide heat for the evaporators. Coadeased steam is returned to the cycle. Such units may be particularly prone to corrosion by salts. Sizes vary, but a plant scheduled for completion in 1996 had six units and a total capacity of 345,600 m /d. Power generation was expected to be 17,500 MW (36). 

Corrosion. Copper and selected copper aHoys perform admirably in many hostile environments. Copper aHoys with the appropriate corrosion resistance characteristics are recommended for atmospheric exposure (architectural and builder s hardware), for use in fresh water supply (plumbing lines and fittings), in marine appHcations (desalination equipment and biofouling avoidance), for industrial and chemical plant equipment (heat exchangers and condensers), and for electrical/electronic appHcations (coimectors and semiconductor package lead-frames) (30) (see Packaging). 

Corrosion resistance is inferior to that of austenitic stainless steels, and martensitic steels are generally used in mildly corrosive environments (atmospheric, fresh water, and organic exposures). 

Galvanic corrosion is more probable in all these various circumstances if the coolant is a conductive fresh water, brackish water, or sea water. 

The annular space between the outer pipe and the surrounding rock is filled with cement over the whole depth up to the ground in new wells. The purpose of this is to seal the deposits at the top and to keep the fresh water and salt water zones separate. In addition, it serves as a protection against pressure from the rock and as corrosion protection which, however, is only effective so long as there is no current exit caused by extended corrosion of cells or due to foreign anodic influences. The cement Ailing of the borehole casing is usually not uniformly spread over the pipe surface. It has to be remembered that there can be sections which are either not covered or are only thinly covered with cement. 

Other examples of metallurgy decisions are red brass versus admiralty tubes with fresh water on the tubeside and suspected stress corrosion cracking conditions on the shellside, and stainless steel versus carbon steel with chlorides present. A good metallurgist should be brought in when these kinds of decisions are needed. 

In the case of a closed water system, once the correct water treatment is provided, the incidence of microbiological fouling or corrosion is virtually eliminated, provided that the addition of fresh water is not a frequent occurrence. It is, however, essential to have water tests carried out at regular intervals by a water laboratory. 

Nickel is usually alloyed with elements including copper, chromium, molybdenum and then for strengthening and to improve corrosion resistance for specific applications. Nickel-copper alloys (and copper-nickel alloys see Section 53.5.4) are widely used for handling water. Pumps and valve bodies for fresh water, seawater and mildly acidic alkaline conditions are made from cast Ni-30% Cu type alloys. The wrought material is used for shafts and stems. In seawater contaminated with sulfide, these alloys are subject to pitting and corrosion fatigue. Ammonia contamination creates corrosion problems as for commercially pure nickel. 

Copper has excellent resistance to some corrosive environments, including fresh waters and fluoride-containing atmospheres. Alloying is necessary to achieve good strength, but copper limiting with steel for strength is an alternative (BS 5624). Copper and some of its alloys are susceptible to crevice corrosion, but the mechanism is different from that which affects stainless steels. 

Cathodic protection (CP) is an electrochemical technique of corrosion control in which the potential of a metal surface is moved in a cathodic direction to reduce the thermodynamic tendency for corrosion. CP requires that the item to be protected be in contact with an electrolyte. Only those parts of the item that are electrically coupled to the anode and to which the CP current can flow are protected. Thus, the inside of a buried pipe is not capable of cathodic protection unless a suitable anode is placed inside the pipe. The electrolyte through which the CP current flows is usually seawater or soil. Fresh waters generally have inadequate conductivity (but the interiors of galvanized hot water tanks are sometimes protected by a sacrificial magnesium anode) and the conductivity... 

Copper and its alloys in certain fresh waters give rise to a form of localised attack that is referred to as nodular pitting in which the attacked areas are covered by small mounds or nodules composed of corrosion products and of CaC03 precipitated from the water. This is a serious problem in view of the extensive use of copper pipes and tanks for water supplies, and in aggressive water these may perforate in a relatively short time. 

Published work relating to bimetallic corrosion in sodium chloride solution is reported in Referencesin sea-water in Refer-ences " in fresh waters in Referencesin mineral acids in References in water/glycol mixtures in Reference ... 

The most important property of the dissolved solids in fresh waters is whether or not they are such as to lead to the deposition of a protective film on the steel that will impede rusting. This is determined mainly by the amount of carbon dioxide dissolved in the water, so that the equilibrium between calcium carbonate, calcium bicarbonate and carbon dioxide, which has been studied by Tillmans and Heublein and others, is of fundamental significance. Since hard waters are more likely to deposit a protective calcareous scale than soft waters, they tend as a class to be less aggressive than these indeed, soft waters can often be rendered less corrosive by the simple expedient of treating them with lime (Section 2.3). 

Corrosive conditions are, of course, much less onerous when contact with fresh water only is involved apd less resistant grades may be used in some instances. 

Nickel-iron alloys suffer significantly less corrosion than mild steel when exposed to a soft, fresh water but Friend found that the resistance to pitting is only slightly greater (Table 3,34). 

Table 3.34 Resistance of Fe-36Ni and mild steel to corrosion in fresh water ...

The corrosivity of a natural water depends on the concentration and type of impurity dissolved in it and especially on its oxygen content. Waters of similar oxygen content have generally similar corrosivities, e.g. well-aerated quiescent sea-water corrodes cast iron at ratesof 0 05-0-1 mm/y while most well-aerated quiescent fresh waters corrode iron at O Ol-O-1 mm/y. 

Those waters in which the carbon dioxide content is in excess of that required as bicarbonate ion to balance the bases present are among the most aggressive of the fresh waters. Hard waters usually, though not invariably, deposit a carbonate scale and are generally not appreciably corrosive to cast iron, corrosion rates of less than 0-02 mm/y being frequently encountered. Water-softening processes do not increase the corrosivity of the water provided that the process does not result in the development of an excess of dissolved carbon dioxide. 

Corrosion problems that arise are frequently discussed under the headings (a) sea-water, and (b) fresh waters, but there is, in fact, no sharp dividing line, since some harbour, estuarine and brackish well waters are mixtures of sea-water and fresh water and are often variable in composition. In the past, corrosion problems were serious, particularly in sea-water service, but resistant alloys have been developed and although trouble still occasionally arises this is more frequently due to poor design or operation rather than to lack of materials suitable for the application. 

There are several distinctive types of corrosion that copper and copper alloys may suffer, particularly in sea-water, but also on occasion in fresh waters. The more important of these are discussed briefly below. 

Fresh waters are, in general, less corrosive towards copper than is sea-water, and copper is widely and satisfactorily used for distributing cold and hot waters in domestic and industrial installations . Copper and copper alloys are used for pipes, hot-water cylinders, fire-back boilers, ball floats, ball valves, taps, fittings, heater sheaths, etc. In condensers and heat exchangers using fresh water for cooling, tubes of 70/30 brass or Admiralty brass are usually used, and corrosion is rarely a problem. 

V (SHE) and the corrosion potential of Zn approximates to this value in sea water, but is more positive in fresh waters, particularly at elevated temperatures. 

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