General or Uniform Corrosion
General corrosion is rather predictable. The uniform attack of an entire area exposed to a corrosive media usually is expressed as an average loss-of-metal-thickness over a given period of time and is expressed 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. These data illustrate the performance of the alloys in a variety of environments and do not necessarily simulate a particular process or industry environment. Note that the AL-6XN alloy has a much lower general corrosion rate than the 300 series stainless steels in these aggressive environments.

Pitting Corrosion
Probably the most important characteristic of a stainless steel alloy exposed to chloride containing solutions is its 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. These data were 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 CPT's 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 a significantly greater resistance to pitting.

Table 2: Critical Pitting Temperatures

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 in an acidified 3.5% sodium chloride solution at room temperature for austenitic stainless steel, ferritic stainless steel, and AL-6XN stainless 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 duringwelding. 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.