by David Crowe and Randy Nixon Corrosion Probe, Inc.

Transcription

1 by David Crowe and Randy Nixon Corrosion Probe, Inc.

2 Introduction to Stainless Steels Austenitic 300 Series 304/304L and 316/316L Are Most Common Face-Centered Cubic Arrangement of Atoms We Call This Microstructure Austenite - Good Ductility Duplex e.g Contains Grains With Two Different Atomic Structures: Face-Centered Cubic and Body- Centered Cubic Ferrite and Austenite About 50/50 It Has Good Strength

3 Surge Tank Built of 2101 Lean Duplex Stainless Steel in the Supply Line to a Municipal Water Treatment Plant

4 Composition of Stainless Steels Type UNS Cr, % Ni, % Mo, % N, % Relative Cost * 304L S L S < SMO S AL6XN N S S S S * - ratios vary with time, product type, quantity 1.96

5 Passivity of Stainless Steels A Surface Oxide Layer Is Highly Corrosion Resistant So, the Steel is Called Passive. The Oxide Forms in the Presence of Air and Water (Vapor). Contaminants Incorporated Into the Oxide Can Impair Its Corrosion Resistance

6 Corrosion of SS in Water Treatment Low Rates of Corrosion in Potable Water In Waters Containing Chlorides Such as Brackish Wells, or Waters with Seawater In- Leakage Localized Corrosion Can Occur: Pitting and Crevice Corrosion

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

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

9 Pitting Resistance Can Be Ranked by the Pitting Resistance Equivalent Number or PREN where PREN= %Cr + 3.3%Mo + 16% N

10 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.

11 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

12 Low ph Can Develop in Condensates, with Chlorides

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 Chloride Stress Corrosion Cracking Austenitic Stainless Steels Such as 304 and 316 Are Susceptible to Cracking When Stressed in Chloride-Containing Waters. Duplex Stainless Steels Are More Resistant!

15 Microbiologically Influenced Corrosion (MIC) More of a Problem in Stagnant or Low Flow Conditions. Communities of Microbes Form Slime. Corrosion Results From Reduction of Sulfur- Containing ions, Reactions of Steel with Metabolic Waste Products Corrosion Products Form a Crust Underneath Which Conditions Can Become Very Acidic and Corrosive. Need to Go to 6% Mo SS or 2507 Duplex Stainless Steel

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

17 MIC of a Valve Casting

18 ASTM Specifications for Wrought Austenitic Stainless Steels Type Plate Pipe Fittings 304L A240, A666 A312, A358, A L A240, A666 A312, A358, A409 AL6XN A240, B688 B675, B690, A312 A182, A403, A774 A182, A403, A774 B366G. 6XN

19 Format of an ASTM Specification

20 ASTM Specifications for Duplex Stainless Steels Type Plate Pipe Fittings 2205 A240 A790, A928 A182, A A240 A790, A928 A276, A A240 A790, A928 A182, A815

21 Wrought Equivalent Specifications for Cast Stainless Steels Cast Grade Specifications 304L CF3 A351, A743, A L CF3M A351, A743, A744 AL6XN CN3MN A743, A CF3M A351, A743, A CD3MN,4a or CD4MCu A351, A743, A744, A890

22 Welding Stainless Steels The Corrosion Resistance of Welds Is Inherently Poorer Than the Same Wrought Product Due to the Cast Nature of the Welds Overmatching Weld Filler Metal Increases the Corrosion Resistance of Welds: Weld 304L with 316L Weld 316L with 317L AL6XN with Alloy 625 or 622

23 SS Weld Corrosion in a Water Treatment Plant

24 Corrosion of This 316L Tank Started at the Welds Which Were Not Over-Matched Composition

25 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.

26 Front Page of a Welding Procedure Specification

27 Post Weld Surface Cleaning Remove All Weld Slag and Heat Tint Mechanical Cleaning Should Not Leave Abrasions Do Not Use Tools That Have Been Used on Carbon Steel These Will Leave a Residue of Carbon Steel, Where Corrosion of the Stainless Steel May Start Chemical Cleaning and Passivation May Be a Final Step Shipping and Handling Must Be Done So That Contamination of the Surface Cannot Occur

28 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.

29 Grinding With Carbon Steel Tools Will Contaminate the Surface

30 Carbon Steel Contamination of Duplex Stainless Steel Tank Wall

31 Type 316 SS Corrosion Due to Chlorine Vapor

32 Design for Success Drainage to Prevent Stagnant Conditions Coating Underneath Insulation to Prevent Stress Corrosion Cracking Under Insulation Allow for Thermal Expansion of Stainless Steel, Which Is Larger Than for Carbon Steel Minimize Connection of Dissimilar Metals to Prevent Galvanic Corrosion

33 Stainless Steel Fasteners Type 304L or 316L Bolts May Fail Due to Chloride Stress Corrosion Cracking (ClSCC) Crevice Corrosion and Pitting May Occur in Marine Environments May Have to Go to a Higher Alloy Such as a 6% Mo Stainless Steel Galvanic Corrosion Between SS Fasteners and Carbon Steel Equipment Is Not Usually a Problem Coated Carbon Steel Bolts Are Often a Better Choice

34 Crevice Corrosion of a 316 SS Bolt in an Outfall Line in Seawater

35 Maintenance & Repair of Stainless Steel Equipment Good Housekeeping Minimizes Under-Deposit Crevice Corrosion, Surface Build-Up of Corrosive Contaminants Weld Repairs Require Clean Conditions. Cracks and Other Flaws Must Be Carefully Removed Before Welding

36 Mahalo