steels stainless of Supporting presentation for lecturers of resistance Architecture/Civil Engineering Corrosion Chapter 05: Corrosion Resistance of Stainless Steels 1 steels Contents stainless of 1. Most materials decay over time 2. Why does stainless steel resist corrosion resistance 3. Types of corrosion of stainless steels Corrosion Corrosion 4. How to select the right stainless steel for adequate corrosion resistance . Structural applications . Other applications 5. References 2 1. Most Most 1. materials decay over time over 3 Corrosion resistance of stainless steels steels Most materials decay over time stainless of Material Wood Steel Concrete resistance Corrosion Corrosion Type of decay Fungi Rust Cracking/ Insects Spalling Sun+rain Mitigating actions Chemicals Galvanising Corrosion resistant Paint/varnish Painting rebar 4 steels Most materials decay over time stainless of Material Stone Glass Polymers resistance Corrosion Corrosion Type of decay Wear Breaks Become brittle Damage by under UV light Pollution Mitigating actions Usually none taken Tempered glass Improved polymer grades 5 steels Most materials decay over time stainless of Material Aluminum* Copper Stainless resistance Corrosion Corrosion Type of decay Pitting over time, Forms a green No decay possible galvanic patina over time corrosion Mitigating actions Galvanic corrosion None None required can be prevented * Aluminum forms a thin protective oxide just like stainless, but with a much lower corrosion resistance 6 Corrosion in concrete steels (corrosion problems are not limited to outside surfaces !) stainless of . Corrosion of unprotected carbon steel occurs even inside reinforced concrete structures as chlorides resistance present in the environment (marine/deicing) diffuse through the concrete. Corrosion Corrosion . Corrosion products (rust) have a higher volume than the metal, Stainless steel create internal tensions causing the provides both strength concrete cover to spall. and corrosion . Mitigating the corrosion of steel resistance inside the reinforcing bar in concrete is a must. concrete, providing a . Various techniques are used: thicker long, maintenance- concrete cover; cathodic protection; free service life of the membranes, epoxy coating … and structure. stainless steel rather than C-Steel. 7 steels stainless of resistance 2. Why does stainless steel resist Corrosion corrosion 8 Passive Layer vs. Coatings steels stainless PASSIVE FILM on STAINLESS STEEL: of Oxy-hydroxides of Fe and Cr Oxygen resistance 2-3 nm thick (0,002-0,003 µm) Transparent Corrosion Adherent Self repairing Topcoat MULTI-LAYER COATINGS Typically Coat 20-200 µm thick Primer May peel off Not self-repairing MILD STEEL 9 steels Damage to protective layer stainless Stainless Steel Mild Steel of Passive film Multi-layer Coating resistance Corrosion Corrosion Self Repair Corrosion Products 10 3. Types of corrosion of of corrosion Types 3. stainless steels 11 Corrosion resistance of stainless steels steels Effect of Chromium Content on Atmospheric Corrosion Resistance (uniform corrosion) stainless of Corrosion Rate mmpy 0.200 resistance Plain Carbon Steel 0.175 0.150 Corrosion Corrosion 0.125 0.100 0.075 Stainless Steels 0.050 > 10.5 Cr % 0.025 0 2 4 6 8 10 12 14 16 % Chromium 12 steels stainless of When the selection of the stainless steel grade has resistance not been properly made, corrosion may occur Corrosion Corrosion …no material is perfect! think of it as selecting the right vehicle for the intended use 13 steels Types of corrosion on stainless steels stainless of a) Uniform b) Pitting resistance c) Crevice Corrosion Corrosion d) Galvanic e) Intergranular f) Stress corrosion cracking See Reference 1 14 steels a) What is uniform corrosion? stainless of . When the passive film is destroyed by the aggressive environment, the whole surface corrodes uniformly resistance and metal loss can be expressed as µm/year . This is typical of unprotected Carbon steels. Corrosion . This does not occur on stainless steels in the building industry, as the corrosion conditions are never that aggressive (it requires typically immersion in acids) 15 steels b) What is pitting corrosion1,2,3,7? stainless of Pitting corrosion, or pitting, is a form of extremely localized corrosion that resistance leads to the creation of small holes in the metal. Corrosion Corrosion This picture shows pitting of stainless steel EN1.4310 (AISI 301) resulting from insufficient corrosion resistance in a very aggressive chlorinated environment. 16 steels Pitting corrosion mechanisms stainless of 1. Initiation on a very small surface irregularities or non-metallic inclusions resistance 2. Propagation as the electrochemical reactions in the pit cavity are not prevented by Corrosion re-passivation 17 Pitting can be reproduced in an electrochemical cell4 steels stainless . Corrosion involves the dissolution of metal, i.e. an electrochemical process with of a) electrochemical reactions at the surface of the metal and b) a current between the corroding metal (anode) and a cathodic part . These processes can be simulated in an electrochemical cell, a device that allows the study of corrosion processes resistance E Electrochemical cell pit Polarization curve Corrosion Corrosion 18 steels Major factors that influence pitting corrosion1 (the pitting potential Epit is generally used as the criterion for pitting) stainless 1. Temperature of resistance Corrosion Corrosion Increasing the temperature reduces drastically the resistance to pitting. 19 steels Major factors that influence pitting corrosion5 (the pitting potential Epit is generally used as the criterion for pitting) stainless of 2. Chloride concentration The pitting resistance decreases a the Cl- concentration increases resistance (the log of the Cl- concentration) Corrosion Corrosion 20 steels Major factors that influence pitting corrosion1 (the pitting potential Epit is generally used as the criterion for pitting) stainless of 3. Stainless steel analysis The pitting resistance increases strongly with some alloying elements: N, Mo, Cr resistance Corrosion Corrosion The role of the alloying elements is described by the PREN (Pitting Resistance Equivalent Number) 21 Pitting Resistance Equivalent Number (PREN)6 steels . By calculating the PREN it is possible EN AISI PREN stainless of to compare stainless steel grades 1.4003 - 10.5 - 12.5 resistance against pitting. The higher 1.4016 430 16.0 - 18.0 the number the better the resistance. 1.4301 304 17.5 - 20.8 . Obviously the PREN alone cannot be 1.4311 304LN 19.4 – 23.0 resistance used to predict whether a particular 1.4401/4 316/L 23.1 – 28.5 grade will be suitable for a given 1.4406 316LN 25.0 – 30.3 application Corrosion 1.4439 317L 31.6 – 38.5 1.4539 - 32.2 – 39.9 PREN = Cr + 3.3Mo + 16N, where 1.4362 - 23.1 – 29.2 Cr = Chromium content 1.4462 - 30.8 – 38.1 1.4410 - 40 Mo = Molybdenum content 1.4501 - 40 N = Nitrogen content Please note that the PREN does not involve Ni. The resistance to pitting corrosion does not depend upon the Ni content of the stainless steel. See next slide 22 PREN of some common grades9 steels stainless of - - - - - - - - - - resistance Corrosion Corrosion Ferritic stainless steels can match 304 and 316 austenitic stainless steels in pitting corrosion resistance. Note: Please see Appendix for EN standards designations 23 steels c) What is Crevice Corrosion1? stainless of Crevice corrosion refers to corrosion occurring in confined spaces to which the access of the working resistance fluid from the environment is limited. These spaces are generally called crevices. Examples of crevices are gaps Corrosion Corrosion and contact areas between parts, under gaskets or seals, inside cracks and seams, spaces filled with deposits and under sludge piles. 24 steels Mechanism of Crevice Corrosion stainless of . Initially, no difference between the cavity and the whole surface resistance . Then things change when the cavity becomes depleted in oxygen . A set of electrochemical reactions Corrosion occurs in the crevice, with the result of increasing Cl- concentration and decreasing the local pH, to the extent that passivation cannot occur . Then the metal in the crevice undergoes uniform corrosion 25 Critical Pitting Resistance Temperature (CPT) Critical Crevice Corrosion Temperature (CCT) steels of various austenitic & duplex grades8 stainless Note: The higher the Temperature, the better the corrosion resistance of resistance Corrosion Corrosion Note: Please see Appendix for EN standards designations 26 steels How to avoid crevice corrosion stainless of 1. Optimize design: a) Use welded parts. resistance b) Design vessels for complete drainage. 2. Clean to remove deposits (whenever Corrosion possible) 3. Select a suitably corrosion resistant stainless steel (see part 4 of this chapter) 27 d) What is Galvanic Corrosion1? steels (also known as bimetallic corrosion) stainless of Corrosion that can occur when 2 metals with very different galvanic resistance potentials are in contact. The most anodic metal is attacked Corrosion Corrosion Example on the picture on the left: The stainless steel plate was secured to a stainless steel vessel, using mild steel bolts – resulting in galvanic corrosion of the bolts in presence of humidity, (=electrolyte) See Reference 11 28 Mechanism of galvanic corrosion steels stainless of . Each metal has a characteristic potential when immersed in an electrolyte (measured against a reference electrode.) . When 2 metals are connected with a conducting liquid (humidity is enough):