Module 2: Chapter 7-11
暂无描述。系统推荐的高质量记忆内容，适合每天坚持背诵学习。
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two dissimilar metals are in closed vicinity & exposed to aqueous solution
GALVANIC CORROSION
ion transfer is high
GALVANIC CORROSION
PREVENTION OF GALVANIC CORROSION *CCSSUE**
• Cathodic protection - sacrificial metal - ZINC • Choose relative surface area, anodic metal > cathodic metal • Separate dissimilar metals w/ non-conducting material • Separate metals from conductive environment • Use of poorly conducting electrolytes (water w/ no impurities) • Eliminate metallic ions - (hastens corrosion)
Crevice corrosion is a type of
PITTING CORROSION
2 Types of Stress Corrosion Cracking
• Chloride Stress Corrosion Cracking • Caustic Stress Corrosion Cracking
occurs where the anodic site becomes fixed in small area
PITTING
the formation of holes (deep attack) in an unaffected area due to localized (high and low) oxygen concentration
PITTING
It causes the corrosion cells, changing as surface and solution differences change
GENERAL OVERALL CORROSION
Presence of ____ promote pitting at areas of metal surface that are initially anodic w/ respect to adjacent area
OXYGEN
If the cells do not shift _____ results
PITTING
A ___ stainless steel tube and tube sheet of HEX suffered crevice corrosion due to its gap
TYPE 316 SWRO Desalination Plant
confined space w/c access of working fluid is limited (low)
CREVICE
It refers to localized attack on metal surface, adjacent to the gap or crevice between 2 joining surfaces
CREVICE CORROSION
___ cause pitting of iron and steel
CHLORIDE IONS
cause HIGHLY LOCALIZED DEFECTS in the passivating oxide layer on metal
CHLORIDE IONS
The ___ ___ will then be very large at anodic site and attack on metal will be rapid.
current density
attack wherein part of metal surface is in shielded or restricted environment, compared to the rest that is exposed to electrolyte
CREVICE CORROSION
a HAZARD due to rapid penetration of metal with little overall loss of mass
PITTING CORROSION
WAYS TO MINIMIZE PITTING CORROSION AUADR
• Avoid stagnant conditions • Use correct metals and alloys (less susceptible to corrosion) • Avoid agents in medium that causes pitting • Design system and components that has no crevices • Remove dissolved gases and chlorides (corrodants)
PREVENTION CHEMISTRY CONTROL PPCRP
• PASSIVATION - Pre-treatment/Pickling • PASSIVATION - Passivators and Inhibitors • Cathodic Protection • Removing Corrosive Agents (Corrodants) -Mechanical -Chemical Addition • pH Correction
involves careful control of coolant water chemistry and temperature during ____
PRE-TREATMENT/PICKLING
when a naturally active metal corrodes at very low rate due to oxide coating
PASSIVATION
chemical substances that are added to water to provide passivation by undergoing reduction at metal surface
PASSIVATORS OR INHIBITORS
common passivator
Pottasium Chromate (K2CRO4)