Corrosion Resistance Performance of Stainless Steel Types
Stainless steel is formulated to resist corrosion. The combination of the alloying elements of chromium, molybdenum and nitrogen in the different compositions of stainless steel increases the resistance to the various types of metal corrosion defined in Table 1 which includes general corrosion, pitting corrosion, crevice corrosion, etc.
The performance of any given type of stainless steel reinforcement will depend on severity of the service environment, expected chloride exposure, pH of the concrete, the quality of the downstream maintenance program pertaining to the facility or structure, and any prior contamination for existing structures. These factors should be considered when specifying the appropriate alloy.
The advantage of the availability of different types of stainless steel with varying compositions and therefore performance is that it allows specifiers/designers to match the chemistry required for the particular service condition.
Pitting Resistance Equivalent Number: PREN
In order to compare on a relative basis the corrosion resistance of the various types of stainless steel, a predictive measurement of a stainless steel's resistance to localized pitting corrosion based on its specific chemical composition has been developed. It’s called the “Pitting Resistance Equivalent Number” or PREN. Basically, a higher PREN-value indicates more resistance of the stainless steel to localized pitting corrosion.
The different austenitic and duplex alloys in A955 provide slightly different levels of corrosion resistance due to the variations in their chemical compositions.
The PREN has become recognized as an indicator of the relative resistance of the various types of stainless steel to corrosion in general. As such, the PREN numbers are a practical and useful tool for ranking and comparing the different grades, but cannot be used to predict whether a particular grade will be suitable for a given application.
Table 2 summarizes the chemical composition of the commonly used types of stainless steel reinforcement as per the current version of ASTM A955 used in the PREN calculation.
Actual or specified range compositions can be used and usually involve chromium, molybdenum and nitrogen in the calculations. Tungsten also appears in some versions of the calculation.
For austenitic steels the predictive expression is :
PREN = %Chromium + 3.3 x %Molybdenum + 16 x %Nitrogen
For duplex steels the predictive expression is :
PREN = %Chromium + 3.3 x %Molybdenum + 30 x %Nitrogen
The PREN values for the stainless steels in ASTM A955 are as indicated in Table 3.
Table 2
Chemical Composition of Common Types
|
UNS Number |
Common Name |
Carbon |
Chromium |
Nickel |
Molybdenum |
Nitrogen |
Manganese |
Phosphorus |
Sulfur |
Silicon |
|
|
|
|
|
|
|
|
|
|
|
|
|
S24000 |
XM-29 |
0.08 |
17.0-19.0 |
2.3-3.7 |
-- |
0.20-0.40 |
11.5-14.5 |
0.06 |
0.03 |
1.00 |
|
S24100 |
XM-28 |
0.15 |
16.5-19.0 |
0.50-2.50 |
-- |
0.20-0.45 |
11.0-14.0 |
0.045 |
0.03 |
1.00 |
|
S31653 |
316LN |
0.03 |
16.0-18.0 |
10.0-13.0 |
2.00-3.00 |
0.10-0.16 |
2.00 |
0.045 |
0.03 |
1.00 |
|
S31803* |
2205 |
0.03 |
21.0-23.0 |
4.5-6.5 |
2.5-3.5 |
0.08-0.20 |
2.00 |
0.03 |
0.02 |
1.00 |
|
S32101 |
2101 |
0.04 |
21.0-22.0 |
1.35-1.70 |
0.10-0.80 |
0.20-0.25 |
4.0-6.0 |
0.04 |
0.03 |
1.00 |
|
S32205* |
-- |
0.03 |
22.0-23.0 |
4.5-6.5 |
3.0-3.5 |
0.14-0.20 |
2.00 |
0.03 |
0.02 |
1.00 |
|
S32304 |
2304 |
0.03 |
21.5-24.5 |
3.0-5.5 |
0.05-0.60 |
0.05-0.20 |
2.50 |
0.04 |
0.03 |
1.00 |
*There are slight differences in the chromium, molybdenum and nitrogen content between these two types of stainless
Table 3
PREN Values
|
Alloy |
Alloy Class |
Chromium |
Molybdenum |
Nitrogen |
PREN (Average) |
|
UNS S24000/XM-29 |
Austenitic |
17.0-19.0 |
-- |
0.20-0.40 |
23 |
|
UNS S24100/XM-28 |
Austenitic |
16.5-19.0 |
-- |
0.20-0.45 |
23 |
|
UNS S31653/316LN |
Austenitic |
16.0-18.0 |
2.00-3.00 |
0.10-0.16 |
27 |
|
UNS S31803/2205 |
Duplex |
21.0-23.0 |
2.5-3.5 |
0.08-0.20 |
36 |
|
UNS S32101/2101 |
Duplex |
21.0-22.0 |
0.10-0.80 |
0.20-0.25 |
30 |
|
UNS S32205/2205 |
Duplex |
22.0-23.0 |
3.0-3.5 |
0.14-0.20 |
38 |
|
UNS S32304/2304 |
Duplex |
21.5-24.5 |
0.05-0.60 |
0.05-0.20 |
28 |
Relative Corrosion Resistance Performance : Laboratory Research
There is an exhaustive body of publicly available research reporting on the corrosion resistance of the various types of commonly specified stainless steel rebar. Typically, research into corrosion resistance of metal reinforcement simulates the electrochemical corrosion process of electron flow from anode to cathode and ions in a reverse flow in various electrolytic laden media (usually NaCl – chloride) in the form of a pore solution or ponded concrete specimens.
Applying various test methods, the induced electrical activity or lack thereof is measured and converted into a corrosion rate which varies according to the intensity of the media and the resistance of the test metal.
Consistently the corrosion resistance research performed over many decades by various independent bodies concluded that stainless steel rebar is vastly superior to other products. The research also concludes that though there may be differences in the relative ranking of the various types of stainless, all types of stainless in the group of A955 products are superior to other products.
Within the group of commonly available and demanded stainless steel rebar products, an average ranking based on the published research of corrosion resistance in descending order is as follows :
UNS S31803 and UNS S31853
UNS S32304 and UNS S32101
UNS S24100