Mils per year

Corrosion rates for each specimen expressed as millimeters (mils) per year... 

The corroded tubercle floor is almost always a dish-shaped depression, much wider than it is deep (Fig. 3.23). Undercutting is very rare. The metal-loss width almost exactly matches the tubercular mound width. Corrosion rates exceeding 50 mil per year are rare, except when tubercles are young. Average local corrosion rates are usually 20 mil per year or less. 

Which Alloy to Use. Unalloyed mild steel parts have been known to corrode at rates as high as 800 mils per year. The low-chrome steels, through 9-Cr, are sometimes much more resistant than mild steel. No corrosion has been reported, with both 2%-Cr and 5-Cr furnace tubes, whereas carbon steel tubes in the same service suffered severe coiTosion. The 12-Cr stainless steels are scarcely, if any, better than the low-chromes. But the 18-8 Cr-Ni steels, without molybdenum, are often quite resistant under conditions of low velocity although they are sometimes subject to severe pitting. 

Also, localized, short-term corrosion rates may be several hundred mils per year (MPY) and very much higher if suitable inhibitors are not included. Consequently, it is vital to employ only well-trained and experienced personnel and to ensure that the appropriate equipment is available on site and is properly used. 

Given that, mils per year (mpy) = 1.437 X mdd/D, where density D = 7.85 for carbon steel. 

Unless otherwise specified, the spring material for multiple spring seals shall be Hastelloy C. The spring material for single spring seals shall be austenitic stainless steel (AISI Standard Type 316 or equal). Other metal parts shall be austenitic stainless steel (AlSl Standard Type 316 or equal) or another corrosion resistant material suitable for the service, except that metal bellows, where used, shall be of the material recommended by the seal manufacturer for the service. Metal bellows shall have a corrosion rate of less than 50 jm (2 mils) per year. 

Express the mils per year corrosion rate in terms of electrical current density for iron (density 7.86 g cm-3 one mil = 0.001 inch = 0.00254 cm). 

For example, a steel pipe is V2 in thick, or 250 mils. Its discard thickness is % in thick, or 125 mils. If the corrosion rate is measured at 25 mils per year, then the expected life of the pipe is 5 years. Corrosion rates in excess of 10 mils per year are normally considered excessive and unacceptable, at least in petroleum refineries. 

This is an electronic method used to measure corrosion rate in mils per year. The corrosion probe can be inserted through a packing gland. It is read periodically with a portable instrument that measures the change in electrical conductivity of the probe. It is simple, but perhaps a little less reliable than the coupon. 

The rate of attack for immersed conditions is fairly uniform in unpolluted sea water and averages about 5 mils per year. General attack, when it occurs, has been observed to be a linear function of time—that is, it continues at a fairly uniform rate, despite an accumulation of corrosion products or marine growth. 

Often, however, steel is subject to pitting attack by sea water. The deepest attack on steel is reported to be about 10 to 15 mils per year. The presence of mill scale on the steel—especially when it covers a large portion of the exposed area—significantly increases the rate of pitting, and penetrations of about 20 mils per year can be expected. This is a result of the mill scale serving as large cathodes to the small, bare anodic areas. Pitting attack often tapers off with continued exposure. 

Thus we note that current density gives a measure of corrosion rate. Corrosion rates are generally expressed in practice as mpy (mils per year), ipy (inches per year), ipm (inches per month) and mdd (loss of weight in milligrams per square decimetre per day), and a nomograph for interconversion of one unit into another unit is depicted in Figure 1.22. 

Corrosion Rale in Ferric Sulfate Test, Mils per year... 

The most common way to report uniform corrosion is in terms of metal thickness loss per unit of time, such as inches per year or millimeters per year. Because uniform corrosion is predictable, even moderately high corrosion rates can be tolerated provided a suitable monitoring and inspection system is utilized. For most chemical process systems, general corrosion rates of less than 2 mils per year (MPY) are acceptable. Rates between 2 and 20 MPY (Imil = 0.001 in.) are routinely accepted as useful engineering materials. In severe environments, rates between 20 and 50 MPY may be economically justified. Rates exceeding 50 MPY are generally not acceptable. 

The success of the Type 300 series stainless steel in molten carbonates is a result of the protective LiCr02 film which forms a compact, tenacious, and self-healing layer. This film forms in about 500 hrs and decreases the corrosion rate to a few mils per year. It has been shown that this film is essentially chromium oxide, with the vacant interstices filled with lithium. Lithium is the only stable ionic species present in the melt which is capable of filling the vacant interstice without expanding the oxide lattice (5). Thus a stable diflFusion barrier is formed which limits further corrosion. 

Galvanized steel plates showed some visual differences between cities, as seen in Figure 8. Some differences were also observed within the same city (e.g. Figure 9). These plates were cleaned using an ammonia wash followed by a chromic acid and silver nitrate dip (ASTM G1). Mils per year penetration rates are shown in Table III. Decreasing corrosivity follows the order ... 

L ASTM A262 practice A Or ASTM A262 practice C No Ditching f- 24 mils per year... 

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