Environments that lead to corrosion

Enivronments

Soils

Oil

Human body

Pollution Control

Pulp and Paper

Soils

Moisture, alkalinity, acidity, compactness (affects permeability of water), oxygen, salts, microorganisms

Microbial Organisms in soils

Deterioration of a metal by corrosion processes that occur directly or indirectly as a result of the activity of living organisms

Anaerobic Bacteria

Bacteria that grow in environments with little or no oxygen

Sulfate reducing bacteria produce sulfides

SO4^2- + 4H2 --> S2- + 4 H2O

Sulfide ( S2-) reduces the cathodic reaction but accelerates the anodic reaction

Dissolution of the metal is accelerated

In case of steel, get iron sulfide

Aerobic Bacteria

Bacteria that grow in environments with oxygen present

Oxidize elemental sulfur to sulfuric acid

2S + 3O2 + 2H2O --> 2H2SO4

Produces localized sulfuric acid concentrations up to 5 wt%, extremely corrosive

Cyclic Growth pattern

Heavy rain reduce the amount of oxygen in soil, so more anaerobic bacteria form

Dry spells reduces the amount of water in the soil allowing for more oxygen, so more aerobic bacteria form

Each of these bacteria produce degradation chemicals, buried steel can quickly de can quickly degrade

Degrade hydrocarbons

 remove a protective asphalt coating

Assimilate iron into the cell walls

resulting in crevice corrosion

Oxidize ammonia into nitric acid

corrodes metal

Produce carbon dioxide

in a water containing environment, creates carbonic acid that corrodes metal

Fungi/mold

Degrade organic matter, producing organic acids

Growth on metal can lead to crevice corrosion

Barnacles and Algae

Attach to solid surfaces and cause crevice corrosion

Also lead to narrowing of openings (like pipes) that cause internal destruction

Affect the quality of the design

Ships lose streamlining, increase power requirements

Oil, Petroleum

Issues with production, transportation, storage, and refining

Carbon dioxide, organic acids, sulfur (H2S), and water

Low temperatures and high temperatures each result in different corrosive behavior

meaning different equipment, different metals, different handling

 

Human Body

Body fluid is aerated (oxygen) with 1% sodium chloride, other salts, and organic compounds at 98F (37C)

Increased corrosion rate

Produce pitting, crevice corrosion, galvanic corrosion, and uniform corrosion

Also produces corrosion byproducts that can cause irritation or infection, can suppress cell reactions, accumulate in organs, interfere with normal functions

Pollution Control

Trying to remove small concentrations of eventually detrimental compounds- SO2, SO3, and HCl

Is these do become more concentrated leading to pitting, crevice corrosion, stress corrosion cracking, erosion corrosion (If in air containing ash)

Pulp and Paper

Most corrosion problems exist in pulp and paper plants

Utilize a large variety of temperature, acids, bases, and water

Large stresses include rotations and blowers

Inhibitors

Changing the electrode potential, metallic coatings

Alter environment

Utilize chemical substances in a solution to increase corrosion resistance

Modify the electrochemical reactions by changing solution

 

Adsorption inhibitors (sitting on surface)

Form a chemisorptive bond with metal surface

Impedes electrochemical dissolution reactions

Majority of organic inhibitors

Film Forming inhibitors

Adsorb onto the surface and then

Passivate the surface by promoting formation of a passive film (Passivating inhibitors)

OR

Precipitate salt products with solubilities greater than solubility of metal ions without (precipitation inhibitors)

 

Adsorption inhibitors

Usually contain N,S, or O atoms

Often used in acidic environments

Form a single monolayer and any additional amount of inhibitor does nothing to help with corrosion

Passivating Inhibitors

Often used in near neutral solutions

Inhibitors makes the oxide film protective or keeps the oxide film protective

React with the oxide surface already present or adsorb and are incorporated into the passive film

Precipitation Inhibitors

React with pores or openings in the film with dissolved cations

Formation of insoluble local precipitates

OR

Exchange with outside ions, replacing them in the film and forming precipitates

Most types of inhibitors would be considered films- Films

defined as one molecule layer thick or a few molecule layers thick

Coatings

Are usually considered at 50 microns or thicker

Paints and Organics

Serve to reduce corrosion by providing a physical barrier to separate the metal from the environment

Can add corrosion inhibitors to the coatings to increase corrosion resistance

Can be penetrated by water, oxygen, and ions (Paints and Organics)

Leads to corrosion beneath the coating

Confined to narrow space between the metal and coating

Degrades the metal, forming corrosion byproducts, that weaken coating adhesion

To get corrosion, need water between coating and metal, as well as oxygen

Water can

1. Diffuse through the coating because of concentration gradient formed by humid environment

2. Diffuse because of osmosis allowing water transport because of an ionic concentration gradient between coating and the environment

3. Enter through voids, pores, or microcracks

Transmission of water through coatings can

 be several orders of magnitude higher than required for corrosion of metal

Water alone doesn't corrode, do need oxygen

Transmission of oxygen through coatings

 much lower than needed for corrosion

Rate limiting step for corrosion would be the transmission of oxygen

Transmission of chlorides through coatings

 is even lower than that of oxygen (negatively charger so positive wants the negative to stay with it)