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Vol. 7
November Issue
Year 2006
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Science Update


in Vol. 7 - November Issue - Year 2006
Corrosion and Galvanic Compatibility for Metal Shot Blasting Media



Author: Heribert Gray

Pros & cons of various metallic media for shot blasting of non-ferrous parts made of Al-alloys or Mg-alloys.

Quality engineering and design requires an understanding of material compatibility. Galvanic corrosion (some times called dissimilar metal corrosion) is the process by which the materials in contact with each other oxidize or corrode. There are three conditions that must exist for galvanic corrosion to occur. First there must be two electrochemically dissimilar metals present. Second, there must be an electrically conductive path between the two metals. And third, there must be a conductive path for the metal ions to move from the more anodic metal to the more cathodic metal. If any one of these three conditions does not exist, galvanic corrosion will not occur. Often when design requires that dissimilar metals come in contact, the galvanic compatibility is managed by finishes and plating. The coating and plating selected facilitate the dissimilar materials being in contact and protect the base materials from corrosion.
However, for the surface preparation by means of air or airless shot blasting with metal abrasives requires the choice of the most suitable media to comply with the galvanic compatibility table in order to avoid corrosive attack of the substrates and work pieces being shot blasted.
Additionally the condition and break-down mechanisms of the operating mixes (dust content and other contaminants) must be kept tidy in order to avoid impingement of angular particles into substrate’s surfaces. These impingements can cause severe corrosive reaction even though the substrate’s surface has been coated or finished.
Filiform corrosion or blistering, a special type of galvanic reaction, is the most common corrosion attack to parts made of Al- & Mg-alloys.
For harsh environments, such as outdoors & for cars, high humidity, and salt environments fall into this category. Typically there should be not more than 0.15 V difference in the "Anodic Index". For example; gold – silver would have a difference of 0.15V being acceptable.
For aluminum die castings the use of Cr-Ni based shot (type AISI 304) will generate more corrosion on the parts itself than Cr-based stainless steel cut wire made of wire in grade AISI 430.
Zinc cut wire will be the more cautious choice if corrosion and intense surface texturing ought to be avoided. Please see table 1.
Al-cut wire (grade AW 1070) will react neutral to most common Al-die casting alloys. For safety parts and other specialties (Al-Mg-Mn – alloys) in the automotive industries, the use of Al-cut wire or Al-shot made of Al Mg5 (Grade AW 5019) has been approved and certified. Since the hardness of this kind of medium is > 120 HV, the shot blasting efficiency is comparable to zinc cut wire (specific weight 6.8 g/cm³) 

Steel shot would work as well from the galvanic corrosion point of view, however the mill scale of the media and the iron is generating a discoloration. The parts  get brownish-rusty due to extraneous corrosion within a short period of time.
For normal environments, such as storage in warehouses or non-temperature and humidity controlled environments. Typically there should not be more than 0.25 V difference in the "Anodic Index".
For controlled environments, such that are temperature and humidity controlled, 0.50 V can be tolerated. Caution should be maintained when deciding for this application as humidity, chloride content and temperature do vary from regions.
Every alloy or metal with higher anodic index than the substrates index can be applied as sacrificial anode, to protect the substrate from corrosion.
The higher the difference in potential the higher the sacrificial effect. On the other hand, every shot blasting media showing a high difference of the galvanic index to the index of the work pieces will generate corrosive products on the parts, even under organic coatings.

MATERIAL LISTING

ANODIC INDEX [V]

Gold, solid and plated, gold-platinum alloy

0

Rhodium plated on silver-plated copper

0.05

Silver, solid or plated; monel metal, high nickel-copper alloys

0.15

Nickel, solid or plated, titanium and its alloys, monel

0.3

Copper, solid or plated; low brasses orbronzes; silver solder; German silvery high copper-nickel alloys; nickel-chromium alloys

0.35

18% Cr-Ni Steel < GRADE 304 pp >

0.35

Brass and bronzes

0.4

High brasses and bronzes

0.45

17% Chromium type corrosion-resistant ferritic steels AISI 430

0.5

Chromium plated; tin plated; 12% chromium type corrosion-resistant steels

0.6

Tin-plate; tin-lead solder

0.65

Lead, solid or plated; high lead alloys

0.7

Aluminum, wrought alloys of the 2000 Series

0.75

Iron, wrought, gray or malleable, plain carbon and low alloy steels

0.85

Aluminum, wrought alloys other than 2000 Series aluminum, cast alloys of the silicon type

0.9

Aluminum, cast alloys other than silicon type, cadmium, plated and chromate

0.95

Hot-dip-zinc plate; galvanized steel

1.2

Zinc, wrought; zinc-base die-casting alloys; zinc plated

1.25

Magnesium & magnesium-base alloys, cast or wrought

1.75

Beryllium

1.85




Author: Heribert Gray
For Information:
Frohn N. A. Inc.
Tel +1.770.819.0089
herb.gray@frohn.com