ON CORROSION IN A MARINE ATMOSPHERE
Last update: 20 August 1996
Note: Many of the notes below come from (are stolen from) two excellent
sources: (1) and (2). This document is a collection of definitions, useful
graphs, references, and venders of corrosion products.
Eight forms of Corrosion
Salt Water Corrosion
Marine Atmosphere Corrosion
Materials and Processes -- notes and
tips for met/ocean applications
References
Venders
comments: reynolds@bnl.gov
EIGHT FORMS OF CORROSION
Corrosion comes in many different forms. Before we can discuss corrosion
in seawater or in a marine atmosphere the different types of corrosion must
be understood.
1. UNIFORM ATTACK. The most common form of corrosion. Characterized
by a chemical or electrochemical reaction that proceeds over the entire
surface or over a large area. Predictable and easily controlled by (1) proper
materials, (2) inhibitors, or (3) cathodic protection.
2. GALVANIC (Two metal). A potential difference exists
between two dissimilar metals when they are immersed n a conductive or corrosive
solution. If the materials are placed in direct contact, this potential
difference produces electron flow between them. Corrosion of the less corrosion
resistant-resistant material is usually increased and attack of the more
resistant material is decreased, as compared with the behaviour of these
metals when they are not in contact.
The less resistant material becomes anodic and the more resistant
material becomes cathodic. Ionized metal atoms leave the anode as
cations, going into solution in the conducting seawater. As the cations
go into solution, free electrons flow in the metal towards the cathode.
The driving force for current and corrosion is the potential developed between
the two metals. The potential differences between metals under reversible,
non-corroding conditions form the basis for predicting corrosion tendencies.
Examples of the galvanic tables are the EMF
SERIES and the GALVANIC SERIES for
commercially available metals and alloys in seaswater.
3. CREVIS CORROSION is an intensive localized corrosion
frequently occuring within crevices and other shielded areas on metal surfaces
exposed to corrosives. This type of attack is usually associated with small
volumns of stagnant solution caused by holes, gasket surfaces, lap
joints, surface deposits, and crevices under bolt and rivet heads.
Combating Crevis Corrosion -- in the splash zoneor ocean environment.
Use welded butt joints instead of riveted or bolted joints
in new equipment.
Close crevices in existing lap joints by continuous welding, caulking,
or soldering.
Design vessels for complete drainage; avoid sharp corners and stagnant
areas.
Frequent inspection and remove deposits frequently.
Use solid non-absorbant gaskets, such as teflon,, whereever possible.
4. PITTING. Pitting is a localized type of attack resulting in
cavities of variable sizes and depths. Initiation sites might be
surface scratches, breaks in protective coatings, variations in surface
composition or finish. Stainless steels exposed to sea water develop
sooner or later pits, especially in locations deprived of oxygen (bottom
or crevices, areas covered with fouling organisms, interior faces of nuts
and threads, etc.)
Pitting is particularly vicious because it is a localized and
intense form of corrosion, and failures often occur with extreme suddeness.
Sometimes the pits take a long time to form -- several months or a year
-- to show up in actual service.
Pits grow in the direction of gravity. Most pits develop and
grow downward from horizontal surfaces. Lesser numbers start on vertical
surfaces, and only rarely do pits grow upward from the bottom of horizontal
surfaces.
Pitting may be considered to be the intermediate stage between overall
corrosion and complete corrosion resistance.
A pit is a unique type of anodic reaction. It is an autocatalytic process.
The corrosion processes within a pit produce conditions which are both stimulating
and necessary for the continuing activity of the pit. The process, once
started, is self-stimulating and self-propogating.
From a practical standpoint, most pitting failures are caused by chloride
and chlorine-containing ions.Seawater and brackish waters are high in
these ions.
Pitting is usually associated with stagnant conditions such as a
liquid in a tank or liquid trapped in a low part of an interactive pipe
system.
Stainless steels are more susceptable to pitting than anyother group of
metals or alloys. The addition of 2% molybdenum to 18-8S (type 304) to produce
18-8SMo (type 316) results in a very large increase in resistance
to pitting. Type 304 is considered un-suitable for seawater use.
Combat pitting by the same methods listed above for crevis corrosion
.
5. INTERGRANULAR COROSSION is characterized by a localized attack
at the grain boundaries of the metal with little deterioration of the crystals
in the grains. The attack is often rapid, penetrating deeply in the metal,
and resulting in early failure. Improper welding can result in high
susecptability to intergranular corrosion.
6. SELECTIVE LEACHING (parting)
7. EROSION
8. STRESS CORROSION. When a metallic part is submitted to repeated
stress cycles of sufficient amplitude, minute cracks develop, grow, and
propogate across the metal until failure of the whole part occurs. When
immersed in a corrosive environment to which the metal is susceptable, the
fatigue dterioration of the metal is accelerated.
A similar effect occurs when a part immersed in a cossosive medium and submitted
to a constant tensile load can deteriorate rapidly in a process called stress
corrosion cracking.
SALT WATER CORROSION
Typical corrosion rates for ordinary steel in the sea is shown in a graph
of CORROSION RATES. We see that the
area in the spalsh zone near the coast has considerably higher corrosion
rates. A big problem with stainless steels is pitting corrosion. A graph
oh PITTING RATES indicates the susecptability
of materials to this kind of corrosion.
MARINE ATMOSPHERIC CORROSION
Atmospheric corrosion is primarily due to moisture and oxygen but is accentuated
by contaminants such as sulphur compounds and sodium chloride (salt spray)
Corrosion of steel on a sea coast is 400 to 500 times greatrer than it is
in a desert. Steel specimens 80 feet from a coast corroded 12 times faster
than those 800 feet away. Salt is the chief contaminant.
Sheltered corrosion is defined as occuring in an area where moisture
condenses or accumulates and does not dry out for long periods of time.
MATERIALS AND PROCESSES
This section lists several very useful materials that will make ones life
easier.
Aluminum Notes: Always use 6061-T6 aluminum or
better grade. Hard anodize is best for exposed equipment. Note
hard anodize adds thickness, so holes and threads must allow for the process.
Hard anodize is NOT conductive, so must be scraped away for a ground connection.
Galvanized Steel: Galvanized steel is not a good material
for oceanographic applications. Heavy structures or mooring lines are made
of galvanized steel because its cost and strength are favorable. However,
it is usually expected to be replaced in a few months. Galvanized steel
is sacrificial to stainless and aluminum. Thus when in contact with these
materials (see the Corrosion table above) it will rust rapidly. Screws
and bolts will become so corroded they cannot be opened.
Self-vulcanizing tape: Use self-vulcanizing as much as possible.
It bonds to itself and makes waterproof (fully submersible) connections
if doen correctly. We use Titon tape (Allied #805-0237). Keep several
rolls of this tape with you at all times.
Stainless Steel: Always use type 316 stainless or a better
marine grade. Avoid mixing two different types of stainless, contact corrosion
will occur. Avoid contact between aluminum and stainless. Actually, avoid
all contact between dissimilar materials. Use nylon/delryn insulators
(sleeves, washers, shims) to isolate dissimilar materials.
Tie-wraps: Always use the black, UV immune types. Avoid
the white ones.
REFERENCES
(1) Fontana, Mars G. (1986) Corrosion Engineering. McGraw-Hill Book
Co., NY, ISBN: 0-07-021463-8, 556 pp.
(2) Corrosion in Oceanography
VENDERS
Corroseal, Inc. 8114 304th Ave. SE, Preston WA 98050, (tel)800-4-NO-RUST
888-466-7878 (fax)206-222-7968 (web)
Makers of Corroseal which converts rust on steel from iron oxide
to a black, inert substance that primes it for a final paint coat. Also
manufactures Corrozyme which is a multi-enzyme solution to remove
salt and eye-invisible oil films from rusted steel surfaces and old painted
surfaces. should be used before Corroseal.
Progressive Products Inc., 4607 Linden Pl., Pearland TX 77584 (tel)713-997-9872)
(fax)713-997-9895 (email)p.oman@ix.netcom.com
Distributer of the Thin Film Technology, Inc. product
line of premium grade solvent-free epoxy coatings.
Plumbing Supply, WEB: www.PlumbingSupply.com
Stainless steel hose clamps -- 100% SS types.
Praxis Technologies, Inc. (web)
Manufacture Prax-Ten coating system for rusted or semi-rusted
surfaces.
Yacht Corrosion Consultants, Inc., Dept Y, 2970 Seaborg Ave., Ventura
CA 93003
Offer test equipment to measure metal bonding, zinc testing
information, coatings for the yachting industry.