Alloy 904L (UNS N08904) is a superaustenitic stainless steel that is designed for moderate to high corrosion resistance in a wide range of process environments. With its highly alloyed chemistry 25% nickel and 4.5% molybdenum, 904L provides good chloride stress corrosion cracking resistance, pitting and general corrosion resistance superior to 316L and 317L molybdenum enhanced stainless steels. Alloy 904L was originally developed to withstand environments containing dilute sulfuric acid. It also offers good resistance to other inorganic acids such as hot phosphoric acid as well as most organic acids.
CORROSION RESISTANCE
General Corrosion
The alloy content of the SS 904L alloy provides a level of general corrosion resistance that is generally higher than familiar Type 316 stainless steel. Illustrative laboratory corrosion data from boiling solutions are listed below.
| Boiling Solution | Corrosion Rate in MPY (mm/a) | |
| SS 316 | SS 904L | |
| 20% Acetic Acid | 0.1(<0.01) | 0.6(0.02) |
| 45% Formic Acid | 10.9(0.28) | 7.7(0.20) |
| 1% Hydrochloric Acid | 226 (5.74) | 21.6(0.55) |
| 10% Oxalic Acid | 40.1(1.02) | 27.1(0.69) |
| 20% Phosphoric Acid | 0.20(<.01) | 0.5(0.01) |
| 10% Sodium Bisulfate | 41.5(1.06) | 8.9(0.23) |
| 50% Sodium Hydroxide | >100(>2.5) | 9.6(0.24) |
| 10% Sulfamic Acid | 63.6(1.62) | 9.1(0.23) |
| 10% Sulfuric Acid | 636(16.2) | 101(2.57) |
Chloride Stress Corrosion Cracking
Resistance to chloride stress corrosion cracking is correlated with nickel content, with the 25% nickel SS 904L alloy, showing improved resistance compared to an 8% nickel alloy like Type 304 stainless steel. Resistance of the SS 904L alloy is further aided by presence of 4.5% molybdenum. However, the alloy is not completely resistant to the extremely severe laboratory test, such as boiling magnesium or lithium chlorides, as shown by the following comparative data.
| Performance Using U-Bend Samples and Test Times to 1000 Hours |
||
| Boiling Solution | SS 316 | SS 904L |
| 42% Magnesium Chloride | Fail | Fail |
| 33% Lithium Chloride | Fail | Fail |
| 26% Sodium Chloride | Fail | Pass |
Fail = Chloride stress corrosion cracks observed. Pass = No cracking observed in testing.
Pitting Corrosion
The SS 904L alloy contains a combination of chromium and molybdenum which produces an improved level of resistance to pitting and crevice corrosion by chlorides compared to SS 316 stainless steel. The alloy may by considered for such end uses as tube-sheets in seawater heat exchangers or pulp and paper bleach plant equipment where a level of resistance above that of 316 is desired but some degree of attack can be tolerated. The SS 904L alloy is not totally resistant to environments such as aerated seawater.
Resistance to chloride ion pitting or crevice corrosion can be ranked in laboratory tests such as 10% Ferric Chloride (ASTM Procedure G-48). In this test, the material is tested with artificial crevices attached, and test temperature is raised after each test period in which no attack is observed. Failure is noted as that temperature at which attack is first observed, called the onset temperature for crevice corrosion attack. In this manner, a series of alloys may be ranked as follows:
| Temperature of Onset of Crevice Corrosion Attack* |
|
| Alloy | °F (° C) |
| SS 316 SS 317 SS 904L |
27 (-3) 35 (2) 65 (18) |
*ASTM Procedure G-48, 10% Ferric Chloride Solution.
Intergranular Corrosion
The low carbon level of the SS 904L alloy provides resistance to intergranular corrosion in the welded condition as judged by laboratory intergranular corrosion rates. ASTM Procedure A708 or A262 Practice E are most commonly applied to the N08904 alloy. The following laboratory tests are illustrative. Corrosion rates are given in mils per year (MPY) and, in parentheses, millimeters per annum (mm/a):
| Inter-granular Corrosion Resistance Test Procedure | Corrosion, Rates Mils per Year, (mm/a) |
|
| SS 316 | SS 904L | |
| Copper-Copper Sulfate- 16% Sulfuric Acid (ASTM A262 Practice E) Evaluated by Bend Test |
No Fissures | No Fissures |
| Ferric Sulfate- Sulfuric Acid (ASTM A262 Practice B) |
35.5 (0.90) |
14.2 (0.36) |
| 65% Nitric Acid (ASTM A262 Practice C) |
34.3 (0.87) |
15.4 (0.39) |
Galvanic Compatibility
The SS 904L alloy may influence corrosion when galvanically coupled to less corrosion resistant materials in relatively high conductivity waters such as seawater. In such a case, the SS 904L alloy, the noble metal, will cause galvanic corrosion of the less noble material. The SS 904L alloy is sufficiently resistant that it may be considered as the tubesheet material for highly corrosion resistant tubing alloys in seawater.
Grade SS 316 is the standard molybdenum-bearing grade. The molybdenum gives Grade SS 316 high corrosion resistant properties and particularly high resistance to pitting and crevice corrosion in chloride environments. Grade 316L is the low carbon version of SS 316. Grade SS 904L is a non-stabilised low carbon high alloy austenitic stainless steel. The addition of copper to this grade gives it greatly improved resistance to strong reducing acids, particularly sulphuric acid. It is also highly resistant to chloride attack- both pitting/ crevice corrosion and stress corrosion cracking. It is also non-magnetic in all conditions. Further, it contains substantial content of the high cost ingredients nickel and molybdenum. Therefore, the higher performance austenitic stainless steels such as SS 904L have very good resistance to corrosion and is sustainable than SS 316.
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