CORROSION

Stress Corrosion                              Corrosion Fatigue                                Fretting Corrosion                              Heat Treatment 

 
 
 
 
 

Protection of dissimilar metal contacts

 

    Certain metals are subject to corrosion when placed in contact with other metals.  This is commonly referred to electrolytic or dissimilar metals corrosion.  Contact of defferent bare metals creates an electrolytic action when moisture is present.  If this moisture is salt water, the electrolytic action is accelerated.  The result of dissimilar-metal contact is oxidation    (decomposition) of one or both metals.  The chart show the metal combinations requirng a protective separator.  The separating materials may be metal primer, aluminum tape, washers, grease, or sealant, depending on the metal involved.

   Contacts Not Involving Magnesium.          All dissimilar joints not involving magnesium are protected by the application of a minimum of two coats of zinc chromate primer in addition to normal primer requirements.  Primer is applied by brush or spray and allowed to air-dry 6 hours between coats.

   Contacts Involving Magnesium.       To prevent corrosion between dissimilar-metal joints in which magnesium alloys is involved, each surface is insulated as follow  :         At least two coats of zinc chromate are applied to each surface.  Next, a layer of pressure-sensitive vinyl tape 0,003-inch thick is applied smoothly and firmly enough to prevent air bubbles and wrinkles.  To avoid creep-back, the tape is not stretched during application.  When the thickness of the tape interferes with the assembly of parts, where relative motion exists between parts, or when service temperatures above 250 F. are anticipated, the use of tape is eliminated and extra coats (minimum of three) of primer are applied.    

 

  

  Forms of Corrosion

Type of Corrosion

Corrosion Control

  Corrosion Removal

Preventive Maint.

  Common Corrosive

List of Agent

Bugs
 
 
 
 
 
 
 
 
 
 
 
 
 
 

 

 Aluminum alloys Cadmium plate Zinc plate Carbon & alloy steel Lead Tin coating Copper & alloys Nickel & alloys Titanium & alloys Chromium & alloys Corrosion resisting steel Magnesium alloys
Al.alloys Yes Yes Yes No No No No No No No No No
Cad.plate Yes Yes Yes No No No No No No No No No
Zinc.plate Yes Yes Yes No No No No No No No No No
Carbon and alloy steel No No No Yes Yes Yes No No No No No No
Lead No No No Yes Yes Yes No No No No No No
Tin.coating No     Yes Yes Yes No No No No No No
Copper and alloys No No No No No No Yes Yes Yes Yes Yes No
Nickel and alloys No No No No No No Yes Yes Yes Yes Yes No
Titanium and alloys No No No No No No Yes Yes Yes Yes Yes No
Cadmium plate No No No No No No Yes Yes Yes Yes Yes No
Corrosion resisting steel No No No No No No Yes Yes Yes Yes Yes No
Magnesium alloys No No No No No No No No No No No Yes
 List of Dissimilar metal contacts, that will result in electrolytic corrosion.
 Red areas indicate dissimilar metal contacts
 
 
 
 
 
Corrosion  of  ferrous  metals.
One of the most familiar types of corrosion is ferrous oxide,..
 
Corrosion  of  aluminum  and  aluminum  alloys.
Corrosion attack on aluminum surfaces is usually quite obvious,
 
Corrosion  of  magnesium  alloys.
Magnesium is the most chemically active of the metals used,.
 
Treatment  of  titanium  and  titanium  alloys.
Attack on titanium surfaces is generally difficult to detect
 
Protection  of  dissimilar  metal  contacts.
Certain metals are subject to corrosion when placed in contact with other metals
 
Processes  and  materials  used  in  corrosion  control
Aircraft parts are almost always given some type surface finish
 
Chemical  treatment.
Parco Lubrizing in a chemical treatment for iron and steel parts
 
 
 
 
 
 
 

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Inspection                                                            Corrosion Prone Areas                                                             Corrosion Limits

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