Dave Crowe and Randy Nixon
Corrosion Probe, Inc.
Commonly Used Stainless Steels
304/304L (18 Cr, 8 Ni)
316/316L (16Cr, 10.5, Ni 2 Mo)
‘L’ stands for ‘Low Carbon’, that is <0.03% C, to
Prevent Depletion of Cr through Formation of
Cr Carbides, Especially During Welding.
Advantage: Much Lower Corrosion Rate Than
Carbon Steel
Protection by Passive Oxide that Forms in Air
Composition of Stainless Steels
Composition, weight %
Type
UNS
Cr
Ni
Mo
N
304L
S30403
18-20
8-12
-
-
316L
S31603
16-18
10-14
2-3
<0.1
254 SMO
S31254
18.5-20.5
17.5-18.5
6-6.5
0.18-0.22
AL6XN
N08367
20-22
23.5-25.5
6-7
0.18-0.25
2101
S32101
21-22
1.35-1.70
0.1-0.8
0.2-0.25
2205
S32205
22-23
4.5-6.5
3-3.5
0.14-0.2
2304
S32304
21.5-24.5
3-5.5
0.05-0.6
0.05-0.6
2507
S32750
24-28
6-8
3-5
0.24-0.32
Cast Versions of Stainless Steels
Slightly Different Composition from Wrought.
Buy According to the Cast Specification.
Wrought
Stainless
304
Cast Equivalent
304L
316
316L
2205
AL6XN
CF3
CF8M
CF3M
CD3MN, 4A
CN3MN
CF8
Microbiologically Influenced Corrosion (MIC)
on a CF3M (316L) Valve Disc in a Water
Treatment Plant
Pitting and Crevice Corrosion
Chloride and Dissolved Chlorine Promote
Breakdown of the Passive Oxide, Leading to
Localized Corrosion by:
Pitting
Crevice Corrosion
Pitting Resistance Equivalent
Number (PREN)
PREN = %Cr +3.3%Mo + 16%N
PREN Values for Various Stainless Steels
SS
Cr, wt%
Mo, wt%
N, wt%
PREN
304L
18
0
0
18-20
316L
17
2.2
0.06
23-28
2304
23
-
0.1
26
2205
22
3
0.15
31-38
2507
25
4
0.3
38-46
AL6XN
20.8
6.4
0.22
43-48
Type 316L Stainless Steel in a Clarifier Suffered
Pitting due to Excessive Chloride Concentrations
Severe Crevice Corrosion of 304 Stainless Steel Occurred
Under a Bolted Plate Interface in a Wastewater
Application Where Chloride Was Present
Critical Temperatures for Pitting and
Crevice Corrosion
There Are Critical Crevice Corrosion
Temperatures, and Critical Pitting
Temperatures, Below Which the Localized
Corrosion is Not a Significant Problem.
The Amount of Certain Alloy Elements
Influences the Resistance. Chromium,
Molybdenum and Nitrogen are Most Important.
Temperatures in the Sun May Easily Reach 120°F
Estimated Critical Pitting Temperatures
in Seawater
Stainless Steel Critical Pitting T,
°F
316L
59
2205
122
2507
183
AL6XN
>195
Threshold Chloride Concentrations for Localized
Corrosion
Threshold Concentrations Depend on the pH & T,
In Typical Water Treatment, at Neutral pH and Ambient
Temperatures, Some Rules of Thumb May be Used:
Type
UNS
Approximate chloride
Pitting
concentration below which Resistance
pitting does not occur
Equivalent
Number
304L
S30403
300 ppm
18
316L
S31603
1000 ppm
23
2205
S32205
1500 ppm
31
317LMN
S31726
2000 ppm
33
AL6XN
N08367
Seawater
43
Chloride Stress Corrosion of 304L Line Under Insulation
After 19 Years Service, Operating at 115 °F about 5 Miles
Inland From the Ocean
Threshold Temperatures for Chloride
Stress Corrosion Cracking
In Near Neutral Waters
Stainless Steel
304L, 316L
2205
6% Mo SS
Temperature Above
Which Cl-SCC is
Likely, °F
120-140
250-285
395
Welds and Localized Corrosion
Cast Structure May Have Poorer
Corrosion Resistance due to
Inhomogeneities of Composition
Passivation May Be Impaired by
Contaminants
Rough Surfaces Hold deposits
Typical Heat Tint at 316L Stainless Steel Welds on
the Exterior of a Tank. This Should be Removed
During Fabrication
Residual Heat Tint Was an
Initiation Site for Pitting
Grinding With Carbon Steel Tools
Will Contaminate the Surface
Microbiologically Influenced Corrosion
(MIC)
Biological Processes May Promote Corrosion
Sulfates May be Reduced to Sulfides (SRB’s), Oxidize
Steel and Produce Highly Acidic Conditions
Iron Depositing Bacteria
Acid Producing Bacteria
Microbial Activity May Increase the Electrochemical
Potential of the SS Surface, Increasing the Risk of
Pitting
Underdeposit (Tubercle) Corrosion of 316L
Stainless Steel
Corrosion Deposit Tubercles May Form, with
Localized Acidic Micro-Environments Inside,
Resulting in Accelerated Corrosion
MIC of 316L Stainless Steel in a Wastewater Clarifier
Initiated at Rough Grinding Gouges Where Microbes
Could Attach to the Surface
Using Stainless Steels Successfully
in Wastewater Environments
Quantify Chloride Concentrations
Carefully Review Process Data – Including Upsets and
Downtime.
Evaporation Above the Waterline Can Cause
Concentrations Several Times Higher in Splash Zones.
Evaporative Concentration May be More Pronounced
in Sunny Places.
Process Additions Such as Chlorine or Ferric Chloride
Will Elevate Chloride Concentration. Ferric Chloride
Will Also Depress the pH in Wastewater. The More
Alkaline Chlorination Will Create a Chlorine Residual.
Concentration Effect Chloride Pitting of Type 316L
Stainless Steel in an Alum Floc Wastewater Clarifier.
Concentrations of Chloride in Deposits Were as High
as 70,000 ppm
Chloride Pitting of
316L Stainless Steel
Launder Trough Just
Above the Waterline
Resulted From
Chloride
Concentration Due to
Wetting and Drying
Chlorides in Crevices
Flanged Connections and Threaded Fasteners
Present Crevice Conditions, where Chlorides
May Concentrate.
Consider Upgrading the Stainless Steel
Fasteners Where Chloride Concentration May
Occur.
This Can Include Fasteners on Buried Lines in
Tidal Zones.
Crevice Corrosion at a Flange, and Pitting on the
304 Stainless Steel Pipe in a Water Treatment
Plant
Stagnant Conditions
Avoid 304L and 316L SS in Stagnant
Conditions.
Consider Non-Metallic Options or More
Highly-Alloyed Stainless Steels.
Type 304L and 316L Tend to Not Suffer from
Localized Corrosion if the Fluid Velocity is
>5 ft/s.
Avoid Galvanic Coupling of
Dissimilar Metals
Connecting Stainless Steels to Carbon Steel or
Aluminum May Result in Galvanic Corrosion.
Small Defects in Coating on Carbon Steel Which is
Connected to a Large Area of Stainless Steel May
Corrode Very Fast (Large SS Cathode, Small CS
Anode). For Example, Coated Carbon Steel Rakes
Connected to SS Suction Tubes in a Wastewater
Clarifier.
May Use Isolation Kits OR Coat the Stainless Some
Distance From The Connection.
Stainless Fasteners on CS Equipment
In Non-Immersion Environments, There Is No
Electrolyte, so No Corrosion.
If Condensation or Wetting Occurs, the Cathodic
Area is So Small that Minimal Corrosion of the
Carbon Steel Occurs.
Marine Environments Result in Chloride-Containing
Surface Contamination.
In Many Cases Coated Carbon Steel Fasteners Are
the Best Alternative.
Overmatch Weld Filler to Minimize
Weld Attack
The Corrosion Resistance of Welds is Generally
Poorer Than the Wrought Base Material.
When Welding 304L, Upgrade to 316L Welds.
When Welding 316L, Upgrade to 317L Welds.
For 6% Mo SS, use Alloy 625 or 622
Always Use L-Grade Weld Filler Metals
A Metallurgical Mount Shows the Result of MIC, with
Subsurface Pitting Damage of 316L Stainless Steel
Adjacent to a Weld
Follow Qualified Welding Practices
Welding Procedure Specification – Lays Out
the Conditions for Welding Successfully –
Having Been Proven by Testing of Welds Made
Accordingly.
Procedure Qualification Record – Documents
the Testing of the Procedure.
Welder Performance Qualification – Individual
Welder Demonstrates Skill by Successfully
Making a Weld.
Post Weld Surface Cleaning
Remove All Weld Slag Residue and Heat Tint.
Mechanical Cleaning Will Aid to Remove Some of
the Surface That Has Been Chromium-Depleted
by the Welding, and Which Would have Poorer
Corrosion Resistance.
Chemical Passivation Will Help to Remove Any
Contamination, Which Could Lead to Corrosion.
Carbon Steel Tools Should Not be Used on
Stainless Steel Because That Would Contaminate
the Surface.
Welds in a Process Water Tank After One Year Where
Heat Tint Had Not Been Removed From the Surface.
Note the Rusting on the Vertical Weld due to Carbon
Steel Contamination During Weld Preparation.
Hydrostatic Testing
Use Potable Water if Possible to <Minimize the
Chloride Concentration. Keep Below 50 ppm
Chloride if Possible.
Drain and Dry Thoroughly, as Soon as Possible After
Successful Completion of Hydrostatic Testing.
To Prevent Stagnant Conditions Where MIC Could
Occur.
Summary
Good Material Selection, Design, Construction
and Operating Practices Will Ensure that the
Investment in Stainless Steel Pays Off.
Well Prepared Specifications
Quality Assurance and Inspection
Mahalo