advertisement

Localized

67 %
33 %
advertisement
Information about Localized
Entertainment

Published on February 18, 2008

Author: Bernardo

Source: authorstream.com

advertisement

Localized Corrosion:  Localized Corrosion Bob Cottis Corrosion and Protection Centre, UMIST This presentation is copyright Bob Cottis 1997. All rights reserved. It may not be copied in any form without prior written permission of the author. Sources of Localized Corrosion:  Sources of Localized Corrosion Environment oxygen concentration chloride ion concentration pH flow rate Sources of Localized Corrosion:  Sources of Localized Corrosion Material segregation inclusions different phases grain boundaries Sources of Localized Corrosion:  Sources of Localized Corrosion Mechanical static stress fluctuating stress Galvanic Corrosion:  Galvanic Corrosion Fe O2 Cu Galvanic Corrosion:  Galvanic Corrosion Important factors in galvanic corrosion relative areas of anode and cathode difference in potential between anode and cathode effect of anodic polarization on anode (some may passivate) Galvanic Corrosion:  Galvanic Corrosion The emf, or electrochemical series. Ordered by standard equilibrium potential Assumes activity of solution species is one Does not allow for protective corrosion products Cannot be defined for alloys The galvanic series. Ordered by observed behaviour in service Allows for corrosion products Can be defined for alloys Different for every environment Galvanic Corrosion - Questions:  Galvanic Corrosion - Questions Which is better, a steel bolt in a brass structure, or a brass bolt in a steel structure? The steel will be the anode, so a small steel bolt in a large brass structure will suffer severe galvanic corrosion. A brass bolt will be protected by the steel structur Which is better?:  Which is better? Brass bolt in a steel structure Steel bolt in a brass structure What should we paint?:  What should we paint? A milk cooler has an inner liner of stainless steel, and an outer casing of mild steel. Chilled brine is used between the steel and stainless steel to cool the milk. Corrosion of the mild steel is occurring - would it be a good idea to paint the inside of the mild steel casing? This would be a very bad idea as any pin-holes in the paint would act as the anode, with the full area of the stainless steel as the cathode. Galvanic Corrosion in Alloys:  Galvanic Corrosion in Alloys Galvanic corrosion can occur between different phases in an alloy This is especially important when one of the phases is much more active as a cathode, e.g.: Cu-containing precipitates in aluminium alloys – initiate pitting corrosion Fe and Cu impurities in commercial zinc – cause a large increase in corrosion rate compared to pure zinc Differential Aeration:  Differential Aeration Fe O2 Fe2+ Fe Crevice Corrosion:  Crevice Corrosion Metal O2 Cr3+ 1 Oxygen is consumed in crevice by slow passive corrosion Metal 2 Passive corrosion continues, and pH falls by Cr3+ hydrolysis 3 Passive film breaks down in acid and rapid active corrosion starts Active corrosion is corrosion occurring in the absence of an oxide film Hydrolysis is a reaction with water, in this case 2Cr3+ + 6H2O  Cr2O3 + 6H+ 4 The active corrosion causes even stronger acidification and stabilises the crevice corrosion Crevice Corrosion:  Crevice Corrosion Crevice corrosion under washers Pitting corrosion on free surface Pitting:  Other anions (e.g. OH- and SO42-) can inhibit pitting, either by buffering the pH in the pit or by causing the precipitation of a salt film Pitting O2 O2 Cr3+ Cr3+ Inside the growing pit the hydrolysis of Cr3+ lowers the pH and breaks down the passive film. The cathodic oxygen reduction reaction continues outside the pit e e Cl- Cl- The presence of chloride is important, as it allows a pH of about 1 to be achieved (HCl is a strong acid, and does not associate) and the metal chlorides are very soluble Characteristics of Pitting Corrosion:  Characteristics of Pitting Corrosion the alloy is passive pitting requires a passive external surface that can provide a high potential to cause the current to flow into the pit; if the external surface is active, this driving force is not available thus carbon steel will only pit if the solution tends to passivate it (e.g. alkaline solutions), it won’t pit if it is corroding generally (e.g. neutral salt solutions) Characteristics of Pitting Corrosion:  Characteristics of Pitting Corrosion passivity broken down locally, usually by chloride the cause of the initiation of pitting corrosion is still not entirely clear, but it involves a very small pit nucleus that grows over periods of the order of seconds Characteristics of Pitting Corrosion:  Characteristics of Pitting Corrosion pits become more stable as they become larger for very small pits the acidity will be neutralised by diffusion into the bulk solution very easily as the pits get larger the diffusion distances increase, and it gets harder for the acidity to diffuse away Characteristics of Pitting Corrosion:  Characteristics of Pitting Corrosion small pits are often stabilised by a film of oxide or metal that partially covers the entrance this allows pits that would not otherwise be stable to continue growing (these are known as metastable pits) Characteristics of Pitting Corrosion:  Characteristics of Pitting Corrosion pitting becomes more likely as the potential becomes more positive this provides a greater driving force for the corrosion process, and helps to stabilise the metastable pits Management of Pitting:  Management of Pitting Stainless steels Pitting breakdown potential (that potential above which the anodic current starts to increase); lower pitting potential implies greater susceptibility Critical pitting temperature (CPT, temperature at which pitting is observed in aggressive conditions); lower CPT implies greater susceptibility Pitting Resistance Equivalent (formula based on composition e.g. Cr + 3Mo + 30 N) gives ranking of alloys Management of Pitting:  Management of Pitting Pit growth tends to follow a law of the form depth = constant x timen where n is between 0.3 and 0.5 Pitting is a statistical process, and can be modelled by extreme value statistics Dealloying :  Dealloying Alloy containing one noble element and one base element (e.g. brass, Cu + Zn) Base element can dissolve leaving a matrix of the noble element dezincification of brass graphitization of grey cast iron Causes loss of mechanical strength without obvious change in shape Dealloying:  Dealloying ‘Plug-type’ dealloying of a brass tube Intergranular Corrosion:  Intergranular Corrosion When stainless steel is heated to about 650oC, Cr carbides form at the grain boundaries Because of its high diffusion rate, carbon can diffuse a long way to form the precipitate However, chromium can only diffuse a short distance, and this reduces the matrix concentration of chromium at the grain boundary If the grain boundary Cr concentration falls low enough (below about 9%), then it will no longer remain passive, and grain boundary corrosion will occur Intergranular Corrosion:  Intergranular Corrosion When grain boundary chromium carbide precipitation has occurred, the stainless steel is said to be sensitised. Sensitisation frequently occurs in the heat-affected zone during welding, and the resultant corrosion is called weld decay. Intergranular Corrosion - Weld Decay of Stainless Steel:  Intergranular Corrosion - Weld Decay of Stainless Steel Solving Weld Decay:  Solving Weld Decay Use a low carbon grade of stainless steel Use a stabilised grade of steel, which includes a strong carbide-forming element (Nb or Ti) to lock up the carbon For existing components that have been sensitised, heat treat to re-dissolve the carbides (but this is very difficult in practice) Flow Effects:  Flow Effects Flow can increase transport of oxygen to the metal surface (may help to passivate) increase the rate of dissolution of corrosion product films mechanically remove oxides Terminology is poorly-defined, and often contradictory Erosion Corrosion:  Erosion Corrosion Corrosion accelerated by impact by solid particles These may remove metal, or they may just remove oxide and allow metal to corrode more quickly Impingement:  Impingement Corrosion rate increased by the impact of turbulent eddies on the metal surface These locally remove the corrosion product by rapid dissolution, and allow rapid active corrosion Characterised by ‘horseshoe-shaped pits’ Cavitation:  Cavitation High velocity flow leads to pressure falling below zero at points of highest flow Vapour bubbles form in these regions as the liquid is ‘pulled apart’ When the pressure increases again the collapse of the vapour bubbles creates an intense shockwave that removes metal or oxide from the surface Flow Effects:  Flow Effects

Add a comment

Related presentations

Related pages

dict.cc Wörterbuch :: localized :: Deutsch-Englisch ...

Englisch-Deutsch-Übersetzung für localized im Online-Wörterbuch dict.cc (Deutschwörterbuch).
Read more

dict.cc | localized | Wörterbuch Englisch-Deutsch

Übersetzung für localized im Englisch-Deutsch-Wörterbuch dict.cc.
Read more

Localized - definition of localized by The Free Dictionary

One-eighth of the whole population were colored slaves, not distributed generally over the Union, but localized in the Southern part of it.
Read more

Localized | Define Localized at Dictionary.com

verb (used with object), localized, localizing. 1. to make local; fix in, or assign or restrict to, a particular place, locality, etc. verb (used without ...
Read more

Localized | definition of localized by Medical dictionary

These helped to break down the isolation of localized communities, as the memories of Anna Tzelniker illustrate: "All the people knew one another, and ...
Read more

Localized Synonyms, Localized Antonyms | Thesaurus.com

Synonyms for localized at Thesaurus.com with free online thesaurus, antonyms, and definitions. Dictionary and Word of the Day.
Read more

Localize - definition of localize by The Free Dictionary

localize Past participle: localized Gerund: localizing Imperative; localize: localize: Present; I localize: you localize: he/she/it localizes: we localize ...
Read more

Localize | Define Localize at Dictionary.com

verb (used with object), localized, localizing. 1. to make local; fix in, or assign or restrict to, a particular place, locality, etc. verb (used without ...
Read more

Localized Apps - Download - heise online

Localized Apps: Täuscht einzelnen Apps eine gewünschte Systemsprache vor, um beispielsweise schlecht übersetzte Apps auf Englisch zu nutzen
Read more

Localized

Localized is dedicated to the creation, execution and promotion of music driven events and activities that raise funds and awareness for local ...
Read more