Stainless steels seawater corrosion

In compounds with other materials, the effect of high-alloyed stainless steels on corrosion in seawater is similar to a great extent. These steels can be connected to one another, and with other materials that are passive in seawater, without a risk of contact corrosion. 

Titanium resists erosion—corrosion by fast-moving sand-laden water. In a high velocity, sand-laden seawater test (8.2 m/s) for a 60-d period, titanium performed more than 100 times better than 18 Cr—8 Ni stainless steel. Monel, or 70 Cu—30 Ni. Resistance to cavitation, ie, corrosion on surfaces exposed to high velocity Hquids, is better than by most other stmctural metals (34,35). 

Vanadium is resistant to attack by hydrochloric or dilute sulfuric acid and to alkali solutions. It is also quite resistant to corrosion by seawater but is reactive toward nitric, hydrofluoric, or concentrated sulfuric acids. Galvanic corrosion tests mn in simulated seawater indicate that vanadium is anodic with respect to stainless steel and copper but cathodic to aluminum and magnesium. Vanadium exhibits corrosion resistance to Hquid metals, eg, bismuth and low oxygen sodium. 

The stainless steels contain appreciable amounts of Cr, Ni, or both. The straight chrome steels, types 410, 416, and 430, contain about 12, 13, and 16 wt % Cr respectively. The chrome—nickel steels include type 301 (18 wt % Cr and 9 wt % Ni), type 304 (19 wt % Cr and 10 wt % Ni), and type 316 (19 wt % Cr and 12 wt % Ni). Additionally, type 316 contains 2—3 wt % Mo which gready improves resistance to crevice corrosion in seawater as well as general corrosion resistance. AH of the stainless steels offer exceptional improvement in atmospheric conditions. The corrosion resistance results from the formation of a passive film and, for this reason, these materials are susceptible to pitting corrosion and to crevice corrosion. For example, type 304 stainless has very good resistance to moving seawater but does pit in stagnant seawater. 

Pitting is a form of localized corrosion in which part of a metal surface (perhaps 1 per cent of the exposed area) is attacked. Rates of pitting penetration can be very high type 316 stainless steel in warm seawater can suffer pit penetration rates of 10 mm per year. This is a natural... 

The electrochemical examination of fusion joints between nine pairs of dissimilar metal couples in seawater showed that in most cases the HAZ was anodic to the weld metals" . Prasad Rao and Prasanna Kumarundertook electrochemical studies of austenitic stainless steel claddings to find that heat input and 5Fe content significantly affected the anodic polarisation behaviour under active corrosion conditions whilst Herbsleb and Stoffelo found that two-phased weld claddings of the 24Cr-13Ni type were susceptible to inter-granular attack (IGA) as a result of sensitisation after heat treatment at 600°C /pa was unaffected by heat input. 

Guide for crevice corrosion testing of iron base and nickel base stainless steels in seawater and other chloride-containing aqueous environments... 

Applications of Rp techniques have been reported by King et al. in a study of the corrosion behavior of iron pipes in environments containing SRB. In a similar study, Kasahara and Kajiyama" used Rp measurements with compensation of the ohmic drop and reported results for active and inactive SRB. Nivens et al. calculated the corrosion current density from experimental Rp data and Tafel slopes for 304 stainless steel exposed to a seawater medium containing the non-SRB Vibrio mtriegens. 

The addition of chromium forms a family of Ni-Cr-Mo alloys such as Hastelloy alloys C-276, C-22, and C-2000. These alloys contain 16 to 22 percent chromium and 13 to 16 percent molybdenum and are very resistant to a wide variety of chemical environments. They are considered resistant to stress-corrosion cracking and very resistant to localized corrosion in chloride-containing environments. These alloys are resistant to strong oxidizing solutions, such as wet chlorine and hypochlorite solutions. They are among only a few alloys that are completely resistant to seawater. The carbon contents are low enough that weld sensitization is not a problem during fabrication. These alloys are also more difficult to machine than stainless steel, but fabrication is essentially the same. 

N. J. E. Dowling, J. Guezennec, and D. C. White. Facilitation of corrosion of stainless steel exposed to aerobic seawater by microbial biofilms containing both facultative and absolute anaerobes. In Proceedings Volume. Inst Petrol Microbiol Comm Microbial Problems in the Offshore Oil Ind Int Conf (Aberdeen, Scotland, 4/15-4/17), 1986. 

R. Gundersen, B. Johansen, P. O. Gartland, L. Fiksdal, I. Vintermyr, R. Tunold, and G. Hagen. The effect of sodium hypochlorite on the electrochemical properties of stainless steels in seawater with and without bacterial films. Corrosion, 47(10) 800-807, October 1991. 

Vanadium is an excellent alloy metal with iron that produces hard, strong, corrosion-resistant steel that resists most acids and alkali. It is even more resistant to seawater corrosion than is stainless steel. Vanadium is difficult to prepare in a pure form in large amounts. Impure forms seem to work as well as a very pure form of the metal when used as an alloy. When worked as a metal, it must be heated in an inert atmosphere because it will readily oxidize. 

Some acids will attack nickel, but it offers excellent protection from corrosion from air and seawater. This quality makes it excellent for electroplating other metals to form a protective coating. Nickel is also an excellent alloy metal, particularly with iron, for making stainless steel as well as a protective armor for military vehicles. It is malleable and can be drawn through dies to form wires. About one pound of nickel metal can be drawn to about 200 miles of thin... 

The most common use of nickel is as an alloy metal with iron and steel to make stainless steel, which contains from 5% to 15% nickel. The higher the percentage of nickel in stainless steel, the greater the steel s resistance to corrosion—particularly when exposed to seawater. Nickel is also alloyed with copper to make Monel metal, which was widely used before stainless steel became more economical and practical. It was used for many purposes as varied as household appliances and general manufacturing. Nickel is also used to electroplate other metals to provide a noncorrosive protective and attractive finish. 

For the following pairs of alloys that are coupled in seawater, predict the possibility of corrosion and if corrosion is possible, note which alloy will corrode (a) Al/Mg (b) Zn/low carbon steel (c) brass/Monel (d) titanium/304 stainless steel and (e) cast iron/315 stainless steel. Clearly state any assumptions you make about compositions of alloys. 

Seawater-based utility systems for condenser and process cooling systems in power plants exhibit serious corrosion, erosion and fouling problems. Equipment made from carbon steel and even stainless steel shows sign of degradation from galvanic effect, corrosion, erosion and microbiological induced corrosion (MIC). Corrosion... 

Stainless steel 316L material used for piping and equipment shows considerable corrosion resistance because of the beneficial effect of molybdenum on the surface properties. It is also observed that the surface treatment (pre-reduced, polished, passivated and chemically treated surfaces) of stainless steel equipment and piping reduces the corrosion process in seawater applications. The corrosion resistance of stainless steel in seawater applications can also be enhanced by bulk alloying the stainless steel with nitrogen, chromium, molybdenum and nickel by converting the stainless steel into super austenitic stainless steel. From leaching studies it is also observed that the release of iron, chromium and nickel from the super austenitic stainless steel to seawater is considerably... 

Materials such as metals, alloys, steels and plastics form the theme of the fourth chapter. The behavior and use of cast irons, low alloy carbon steels and their application in atmospheric corrosion, fresh waters, seawater and soils are presented. This is followed by a discussion of stainless steels, martensitic steels and duplex steels and their behavior in various media. Aluminum and its alloys and their corrosion behavior in acids, fresh water, seawater, outdoor atmospheres and soils, copper and its alloys and their corrosion resistance in various media, nickel and its alloys and their corrosion behavior in various industrial environments, titanium and its alloys and their performance in various chemical environments, cobalt alloys and their applications, corrosion behavior of lead and its alloys, magnesium and its alloys together with their corrosion behavior, zinc and its alloys, along with their corrosion behavior, zirconium, its alloys and their corrosion behavior, tin and tin plate with their applications in atmospheric corrosion are discussed. The final part of the chapter concerns refractories and ceramics and polymeric materials and their application in various corrosive media. 

Many metals are resistant to corrosion due to a compact adherent oxide film which acts as a barrier between the metal and its environment. Aluminium and stainless steels are examples of such metals. Often, the properties of such a barrier film will depend on the environment in which the metal is situated. For example iron in seawater does not produce corrosion products which are protective but in dilute carbonate solutions with no or very low chloride concentrations, a passive complex Fe(0H)2/FeC03 layer is formed [19, 20]. 

IMaterials and Scaling Issues. Two aspects of the basically simple desalination process require special attention. One is the high corrosivity of seawater, especially pronounced in the higher temperature distillation processes, which requires the use of corrosion-resistant, and therefore expensive, materials. Typical materials in use are copper—nickel alloys, stainless steel, titanium, and, at lower temperatures, fiber-reinforced polymers and special concrete compositions (39). It is noteworthy that in quest of a lower initial cost, the use of inadequate materials of constmction in many locations combined with poor operation by virtually untrained hands led to rapid deterioration and failure of plants long before their estimated design life. Adequate experience suggests by now how to avoid such failures. The other aspect is scale formation (40,41), discussed in mote detail below. 

G. N. Flint, R. N. Cox, The resistance of stainless steel partly embedded in concrete to corrosion by seawater , Magftzine of Concrete Research, 1988, 40, 13 27. 

For example, it is not desirable to have a small anode connected to a large cathode as this favors accelerated localized anodic dissolution. Rivets of copper on a steel plate and steel rivets on a copper plate on immersion in seawater for a period of 15 months resulted in the steel plate covered with corrosion products while the steel rivets were corroded completely and disappeared. As copper is more noble than iron, it accelerated the hydrogen reduction reaction for the oxidation of the steel plate. In the case of the copper plate with steel rivets, the steel rivets corroded because of the relatively important cathodic surface of copper. The same reasoning applies to the corrosion of noncoated auto parts in contact with a large stainless steel surface (Fig. 1.6). 

Bimetallic corrosion and other forms of corrosion continued to cause service failures. In 1962, a report was sent to the British Ministry of Defense stating that a copper alloy end plate had fallen off a seawater evaporator in a submarine because the steel bolts with which it was secured had effectively dissolved through galvanic action. In 1982, the nose wheels failed on two Royal Navy Sea Harriers that had returned from the Falklands War. Studies showed that the galvanic action was responsible for the corrosion that occurred between the magnesium wheel alloy and its stainless steel bearing. 

The oxidizer, chlorine, may in larger concentrations lead to breakdown of the passive oxide on stainless steels and cause heavy corrosion. This has happened around swimming pools, where evaporation has led to enrichment of CI2 in water films. In smaller concentrations it is beneficial, preventing organic growth, and chlorination (addition of sodium hypochlorite) is used for this purpose in seawater pipe systems. 

Figure 6.16 shows how different concentrations of chlorine can affect cathodic overvoltage curves for stainless steel in seawater. The corrosion risk for stainless steel at higher CI2 concentrations arises because the increased cathodic reaction rate lifts the potential so that critical potentials for local corrosion arc exceeded (see also Section 8.3). Chlorine may cause corrosion on several other materials as well. 

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