Dave Crowe and Randy Nixon Corrosion Probe, Inc.

Transcription

1 Dave Crowe and Randy Nixon Corrosion Probe, Inc.

2 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

3 Composition of Stainless Steels Composition, weight % Type UNS Cr Ni Mo N 304L S L S < SMO S AL6XN N S S S S

4 Cast Versions of Stainless Steels Slightly Different Composition from Wrought. Buy According to the Cast Specification. Wrought Stainless Cast Equivalent 304 CF8 304L CF3 316 CF8M 316L CF3M 2205 CD3MN, 4A AL6XN CN3MN

5 Microbiologically Influenced Corrosion (MIC) on a CF3M (316L) Valve Disc in a Water Treatment Plant

6 Pitting and Crevice Corrosion Chloride and Dissolved Chlorine Promote Breakdown of the Passive Oxide, Leading to Localized Corrosion by: Pitting Crevice Corrosion

7 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 L AL6XN

8 Type 316L Stainless Steel in a Clarifier Suffered Pitting due to Excessive Chloride Concentrations

9 Severe Crevice Corrosion of 304 Stainless Steel Occurred Under a Bolted Plate Interface in a Wastewater Application Where Chloride Was Present

10 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

11 Estimated Critical Pitting Temperatures in Seawater Stainless Steel Critical Pitting T, F 316L AL6XN >195

12 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 concentration below which pitting does not occur 304L S ppm L S ppm S ppm LMN S ppm 33 AL6XN N08367 Seawater 43 Pitting Resistance Equivalent Number

13 Chloride Stress Corrosion of 304L Line Under Insulation After 19 Years Service, Operating at 115 F about 5 Miles Inland From the Ocean

14 Threshold Temperatures for Chloride Stress Corrosion Cracking In Near Neutral Waters Stainless Steel Temperature Above Which Cl-SCC is Likely, F 304L, 316L % Mo SS 395

15 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

16 Typical Heat Tint at 316L Stainless Steel Welds on the Exterior of a Tank. This Should be Removed During Fabrication

17 Residual Heat Tint Was an Initiation Site for Pitting

18 Grinding With Carbon Steel Tools Will Contaminate the Surface

19 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

20 Underdeposit(Tubercle) Corrosion of 316L Stainless Steel Corrosion Deposit Tubercles May Form, with Localized Acidic Micro-Environments Inside, Resulting in Accelerated Corrosion

21 MIC of 316L Stainless Steel in a Wastewater Clarifier Initiated at Rough Grinding Gouges Where Microbes Could Attach to the Surface

22 Using Stainless Steels Successfully in Wastewater Environments

23 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.

24 Concentration Effect Chloride Pitting of Type 316L Stainless Steel in an Alum FlocWastewater Clarifier. Concentrations of Chloride in Deposits Were as High as 70,000 ppm

25 Chloride Pitting of 316L Stainless Steel Launder Trough Just Above the Waterline Resulted From Chloride Concentration Due to Wetting and Drying

26 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.

27 Crevice Corrosion at a Flange, and Pitting on the 304 Stainless Steel Pipe in a Water Treatment Plant

28 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.

29 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.

30 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.

31 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

32 AMetallurgical Mount Shows the Result of MIC, with Subsurface Pitting Damage of 316L Stainless Steel Adjacent to a Weld

33 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.

34 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.

35 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.

36 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.

37 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