The chloride ion is one of the most corrosive chemical species there is. In iron-base alloys like carbon steel or stainless steel, the chloride ion increases corrosion by increasing the water’s conductivity and by penetrating the alloy’s protective oxides. It can increase overall or general corrosion, and can cause localized corrosion such as pitting aand stress corrosion cracking. The chloride ion occurs naturally in sea water, in ground waters, in potable water and many other bulk liquids.

Aqueous corrosion of steel may be the most common example of corrosion there is, but is the among the most complicated. In analyzing chloride effects, the variables include the chloride level, oxygen content of water, depth in the water, cations in the water, presence of chemical corrosion inhibitors and relative age of steel (new steel may be active or anodic relative to old rusted steel and therefore preferentially corroded).

The corrosion rate increases up to about 3% NaCl, then decreases as the oxygen solubility in the water decreases. Note thet the potassium chloride is more corrosive than sodium chloride while lithium chloride is less so. In quiet sea waters, which around the globe contain 3.15% chlorides steel fully submerged at ambient temperatures corrodes at about 76-305 micron/year in overall corrosion. Up in the splash zone, where air can supply more oxygen, the overall corrosion rate increases to 380 m /yr, and the pitting rate is around 1140 m /yr. Seawater has a pH of 7.8-8.1, but in stagnant harbour basins, bacterial hydrogen sulfide formation may lower the pH to 7, which noticeably increases corrosion. At saturation brines are much more less corrosive, due to the lower oxygen solubility. Flow rate and temperature increases greatly change the corrosion of steel in chloride solutions. Stagnant conditions are always harmful, and below 1.5 m/s, such forms form of corrosion as pitting, oxygen concentration cells, and microbiologically influenced corrosion (MIC) can occur, as well as biological fouling. At flow velocities much above 2.4 m/s, the protective rust or oxide on a steel begins to be stripped off, causing erosion corrosion at rates of 760-1270 m /yr. Temperature increases also accelerates corrosion. In water with low chlorides, temperature serves to drive off oxygen, which may alleviate corrosion, but with strong chloride solutions oxygen may not be so critical. By about 65°C the corrosion rate of steel in chloride solutions is generally over 1279 m /yr. The pH ahs an effects also. Acidification by additions of hydrochloric acid will give an abrupt increase in corrosion rate at a pH of 4, where the rate controlling the reaction switches from the slow diffusion of oxygen through layer of ferrous hydroxide to a rapid reduction of hydrogen ions to give bubbles of hydrogen gas. Alkaline conditions at a pH values above 9 generally slow down the corrosion of steel in water, except if hypochlorites are present in amounts much above 1000 ppm. It is important to remember that most chemical corrosion inhibitor systems designed to control the corrosion of steel in water have limits on how much chloride they can successfully handle.

Chloride attack is the Achilles hell of many common stainless steel, causing pitting, crevice corrosion, and stress corrosion cracking (SCC), especially at elevated temperatures. Both 304 and 316 SS pit deeply in seawater (pH=8) at low flow velocities, even though type 316 contains 2% Mo. When the chloride ion penetrate s the protective oxide on a stainless steel surface, the exposed metal area becomes a small anode that that is selectively and rapidly attacked in an electrochemical corrosive reaction driven by the large surrounding cathodic area of oxide-covered surface. The resulting pit usually grows directly through the metal to produce a narrow, straight side cylindrical pit. The direction of pitting is commonly downwards, so if inspecting a vessel, concentrate on bottom to detect pitting. For types 304 and 316 SS in NaCl solutions at moderate temperatures, pitting is at a maximum at 3.5-4% NaCl. Crevices are especially susceptible of corrosion, because they are deprived of oxygen and therefore anodic and preferentially corroded compared with adjacent surfaces exposed to the air. Experiments have shown that in tight crevice between nonmetallic washers and a 300 series stainless steel, measurable chloride corrosion can occur in waters at chloride levels as low as 100 ppm, while 300 ppm causes an order of magnitude more penetration. Various elements in stainless steel improve chloride resistance. Molybdenum, chromium, and nitrogen help stainless steel resist chloride pitting and crevice corrosion, while the nickel contents determines an alloy’s resistance to SCC. The effects of Mo, Cr and N upon pitting and crevice corrosion in laboratory experiments have been mathematically expressed in an empirical parameter known as pitting resistance equivalent (PRE). The PRE may be plotted vs. critical pitting temperatures (CPT) for various alloys. High values of CPT indicating less susceptibility to pitting and crevice corrosion. In many plots, the CPT varies linearly with the PRE, which is another way of saying that the more one spend for alloy content, the more pitting resistance the alloy has. In the PRE calculation, there are varying coefficients for the elements developed by the various investigators, depending upon the corrosion test used. A generalised formula for the PRE is:

PRE=Cr+(3.0-3.3)Mo+(12.8-30.0)N

Where the symbol for the alloy element signifies its weight percent in the alloy. Ranges are given for the coefficients in formula since different researchers report different values. Stress Corrosion Cracking (SCC) resistance of iron-chromium to chloride varies with the nickel content. The minimum resistance is at the nickel level of 8-12%, the range of 304-316 SS. Only 40 ppm chloride in water can cause SCC of 304/316 SS at 80°C after 1.5-2%. When inspecting vessels for chloride SCC, concentrate on weld areas, where residual tensile stresses can cause the fine, branched cracks characteristic of SCC. this susceptibility of 304/316 SS has led to newer stainless steels with either lower or higher nickel contents. The higher contents, 23.5-42% Ni, have evolved into the so-called super stainless steels. Alloys with lower contents include the high purity ferritic alloys with nickel contents below 1% and the two-phase or duplex alloys containing about 4.5-6.5% Ni. Nickel content determinates the crystal structure of the alloy. Elemental iron ha has a body-centered cubic structure (BCC) at a room temperature, a structure that suffers embrittlement at a low temperatures, but which resists chloride SCC. Adding chromium to impart the stainless quality to the alloy does nor change this crystal structure, but adding nickel does. Nickel has a ductile face-centered cubic (FCC or austenitic) structure, like silver or copper. In the types 304 or 316 SS, the nickel contents produces an alloy that is about 96% FCC and 4% BCC. Alloy AL-6XN (23.5-25.5% Ni, 20.5% Cr, 6-7% Mo, 0.18-0.25% N) is one of the super stainless steels and it is fully FCC structure. In the Duplex stainless steels the nickel content produce alloys that contains about 50% BCC and 50% FCC structure (50% ferrite and 50% austenite). When properly fabricated and welded these alloys have good resistance to moderate chloride service. Other alloy systems used for chloride service include titanium, copper and nickel-based alloys. Titanium can be used in aqueous chloride solutions, the limitations are crevice corrosion at low pH values and hydrogen enbrittlement at high pH values. Copper and its alloys have long be used in saltwater applications, pure copper is useful in salt water for its thermal conductivity, but vulnerable to erosion corrosion at flow velocities much above 0.6-0.9 m/s. 90/10 cupronickel can be an improvement, but questionable if sulphur compounds are present. Nickel base alloys are expensive and are used primarily in strong acid, strong alkali, or complex corrosive services at high temperature. Plastics are generally more resistant to chloride solutions and hydrochloric acid than metals, except for highly oxidising solutions, which may affect some polymers. Natural rubber and synthetic elastomers provide excellent resistance to chloride solutions and even hydrochloric acid.