Corrosion Resistance of AL-6XN Alloy

General or Uniform Corrosion

General corrosion is the uniform attack of an entire area exposed to a corrosive media. The effect is usually expressed as an average loss-of-metal-thickness over a given period of time. Typically the results are described in units such as mils (0.001 inch) per year, or mpy. Table 1 compares the immersion corrosion resistance, conducted in accordance with ASTM G-31, of five alloys in eight different boiling acid and alkali solutions. This information illustrates the performance of the alloys in a variety of environments and does not necessarily simulate a particular process or industry environment. Note that AL-6XN alloy has a much lower general corrosion rate than 300 series stainless steels in these aggressive environments.

Table 1: Corrosion Resistance in Boiling Solutions
Rate ASTM G-31 Test Solution (Boiling) Corrosion Rate in Mils Per Year (mm/y)
Type 316L Type 317L Alloy 904L AL-6XN Alloy 276
20% Acetic Acid 0.12 (0.003) 0.48 (0.01) 0.59 (0.02) 0.12 (0.003) 0.48 (0.01)
45% Formic Acid 23.41 (0.60) 18.37 (0.47) 7.68 (0.20) 2.40 (0.06) 2.76 (0.07)
10% Oxalic Acid 44.90 (1.23) 48.03 (1.14) 27.13 (0.69) 7.32 (0.19) 11.24 (0.28)
20% Phosphoric Acid 0.60 (0.02) 0.72 (0.02) 0.47 (0.01) 0.24 (0.006) 0.36 (0.009)
10% Sodium Bisulfate 71.57 (1.82) 55.75 (1.42) 8.88 (0.23) 4.56 (0.12) 2.64 (0.07)
50% Sodium Hydroxide 77.69 (1.92) 32.78 (0.83) 9.61 (0.24) 11.4 (0.29) 17.77 (0.45)
10% Sulfamic Acid 124.3 (3.16) 93.26 (2.39) 9.13 (0.23) 9.36 (0.24) 2.64 (0.067)
10% Sulfuric Acid 645.7 (16.15) 298.3 (7.58) 100.8 (2.53) 71.9 (1.83) 13.93 (0.35)

Pitting Corrosion

Probably the most important characteristic of a stainless steel alloy exposed to chloride-containing solutions is the resistance to pitting and crevice attack. The pitting resistance of an austenitic stainless steel may be correlated to alloy composition in terms of the Pitting Resistance Equivalent Number. PREN = %Cr + 3.3(%Mo) + 16(%N); where chromium, molybdenum and nitrogen are in weight percent. Increasing the molybdenum in the alloy produces greater resistance to pitting. Therefore, high molybdenum-high chromium alloys generally provide the best pitting resistance. Figure 1 shows the relationship of pitting, molybdenum content, pH, and chloride content.

Figure 1: Pitting Corrosion Relationship as a Function of Chloride, pH, and Molybdenum Contents

Another important consideration is the chloride pitting potential of stainless steel. This is an indication of the susceptibility of the alloy to localized corrosion. If the potential is more positive, the chances of pitting are reduced. Figure 2 indicates that the chloride pitting resistance of AL-6XN stainless steel is far superior to Type 316L stainless steel. This data was obtained from anodic polarization tests conducted in accordance with ASTM G-61 at a scan rate of 1.2V/hr.

Figure 2: Pitting Potential in 3.5% NaCl Solutions

The Critical Pitting Temperature (CPT) is the minimum solution temperature at which pitting is first observed. As shown in Table 2, the CPTs of several stainless alloys were tested in accordance with ASTM G-48B and ASTM G-48A in a test solution containing 4% NaCl + 1% Fe3(SO4)3+ 0.01 M HCl. When compared to the other alloys in these tests, the AL-6XN alloy demonstrated significantly greater resistance to pitting.

Table 2: Critical Pitting Temperatures
Product CCCT1 CPT2 CPT3
°F °C °F °C °F °C
304 <27.5 <-2.5        
316 27.5 -2.5 59 15    
317 35 1.7 66 18.9 77 25
904L 68 20 104 40 113 45
AL-6XN 110 43 177 80.5 172 78
1. Based on ASTM G-48B (6% FeCl3 for 72 hours with crevices)
2. Based on ASTM G-48A (6% FeCl3 for 72 hours)
3. Test Solution: 4% NaCl + 1%Fe3(SO4)3 + 0.01M HCl

Increasing the acidity (decreasing the pH), of a solution beyond a certain value may result in a dramatic increase in the general corrosion rate. This value is referred to as the "depassivation pH," above which the rate is low and below which the rate is high. Corrosion rates for austenitic stainless steel, ferritic stainless steel, and AL-6XN stainless in an acidified 3.5% sodium chloride solution at room temperature show that AL-6XN alloy is the most resistant of the austenitic stainless alloys. The AL-6XN alloy corrosion rate does not appreciably increase until the solution pH falls below 0.3.

Crevice Corrosion

Crevice corrosion is another form of localized corrosion that occurs when the corroding metal is in close contact with anything that makes a tight crevice. Metal degradation at the mating surface of a sanitary clamp fitting and gasket is usually the result of crevice corrosion. Crevice corrosion is usually the first to occur and is predictable as to when and where it will take place. Like pitting, the presence of chlorides makes the reaction proceed at a fast rate. There is a "critical crevice corrosion temperature" (CCCT) below which corrosion will not occur. Figure 3 is a plot of the PREN versus CCCT and metallurgical category. The greater the difference between the CCCT and the operating temperature, the greater the probability that crevice corrosion will occur.

Figure 3: Critical Crevice Corrosion
Temperature as a Function of the
PRE Number

Intergranular Corrosion

The most common example of intergranular corrosion is the formation of chromium carbide in the heat-affected zone (HAZ) of higher carbon stainless steel during welding. These carbides form along the grain boundaries. Because the carbides require more chromium than is locally available, the carbon depletes chromium from the area around the carbon. The grain boundary zone is left low in chromium and creates a new, low chromium alloy in that region. A mismatch in galvanic potential between the base metal and the grain boundary results, so galvanic corrosion begins. As the grain boundaries corrode, the grain and the chromium carbides drop out like particles of rusty sand. The surface of the metal develops a "sugary" appearance.

Intergranular corrosion also can occur whenever intermetallic compounds such as chi or sigma phase form. These compounds usually form when some type of heating occurs, such as welding, heat treatment, or metal fabrication. Understanding how they form makes it relatively easy to control their formation. Since AL-6XN stainless has low carbon, chromium carbide formation usually is not a problem. However, chi phase may be a problem as it forms when the weld metal cools after welding, especially in the heat affected zone, or if heat treatment is improperly performed, or if the alloy is held for a short time in the 1200 - 1800º F (650 - 1000º C) range.

Stress Corrosion Cracking

Because the AL-6XN alloy has increased resistance to SCC it has been used successfully in applications such as chemical process equipment, brewery equipment, feed-water heaters, and flue gas reheaters. AL-6XN alloy is very resistant to SCC at temperatures less than 121°C. The threshold temperature for initiating SCC decreases with increasing chloride content.

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