Do I have to worry about Bimetallic corrosion corrosion in this compression fitting?


The black mark is the result of a careless plumber nearly setting the kitchen on fire when installing that cutoff valve. If your place hasn't burned down by now, you can ignore the burnt spot.

The green discoloring is oxidation caused by a slow leak higher up on the pipe. Tiny leaks sometimes seal themselves via corrosion, so if you don't find any moisture on the pipe, this is another thing you can ignore.

If you want more peace of mind, you could clean the green oxidation from a visible section of the pipe and see if it reappears in a few weeks. Clean pipe = no worries.

If you want less peace of mind, consider this: if the installing plumber was careless enough to burn the wooden cabinet with his torch, and to leave a leak causing that much visible corrosion, what else might he have done incompetently? You may have sloppy plumbing throughout your house.

Welcome to your new home, and good...

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Corrosion is the deterioration of a metal as a result of chemical reactions between it and the surrounding environment. Both the type of metal and the environmental conditions, particularly gasses that are in contact with the metal, determine the form and rate of deterioration.

Do All Metals Corrode?

All metals can corrode. Some, like pure iron, corrode quickly. Stainless steel, however, which combines iron and other alloys, is slower to corrode and is therefore used more frequently.

All small group of metals, called the Noble Metals, are much less reactive than others. As a result, they corrode rarely. They are, in fact, the only metals that can be found in nature in their pure form. The Noble Metals, not surprisingly, are often very valuable. They include copper, palladium, silver, platinum, and gold.

Types of Corrosion

There are many different reasons for metal corrosion. Some can be avoided by adding alloys to a pure metal. Others can be...

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Stainless steel and galvanized materials often are found together in industry with applications such as galvanized fasteners, stainless steel pressure vessels and roof and siding panels. Do I need to worry about these two metals corroding each other? What other concerns should I have pertaining to hot dip galvanized steel and stainless steel in contact?

Stainless steel and galvanized materials often are found together in industry with applications such as galvanized fasteners, stainless steel pressure vessels and roof and siding panels. The presence of two dissimilar metals in an assembly is not always a sign of trouble but it could be a problem. When two metals are in direct contact, there is the potential for the formation of a bimetallic couple. There are four elements necessary for the contact metals to experience corrosion;

One of the metals must act as the anode and generate electrons that can create an electrical current flow. The other metal must act as a cathode...
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Bimetallic corrosion can only occur when two dissimilar metals are in 'electrical' contact and are bridged by an electrically conductive liquid.
The 'cell' produced can result in corrosion to one of the paired metals. This can be an issue when stainless steels are in contact with other metals, depending on the circumstances.

What is needed to set up the corrosion 'cell'?

To set up a galvanic cell between two conducting materials (metals or graphite), the two metals must have differing potentials or be more or less 'noble' than each other.
The more noble metal (cathode) is protected as the less noble metal (anode) sacrificially corrodes.

The table below is an example of these 'metal to metal' relationships, including graphite as conductive non-metal.

The further apart the metals are, in terms of relative potentials, the greater the driving force in a cell. So, for example, stainless steel in contact with copper is less...

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by: Trethewey & Chamberlain, 1988

In 1763 a report fell upon a polished oak desktop in an office of their Lordships of the Admiralty in London. Its concise and detailed script, though of only modest scientific significance at the time, was one of the earliest examples of a solution to a practical engineering problem caused by corrosion. In common with many similar studies carried out since then, however, the essence of its conclusions has been frequently disregarded by engineers throughout the world. (reference)

Two years before the appearance of the report the 32-gun frigate, HMS Alarm, had had its hull completely covered with a thin copper sheathing. The purpose of the sheathing was twofold. Firstly it was intended to reduce the considerable damage caused by the teredo woodworm, and secondly the well-established toxic property of copper was expected to lessen the speed-killing barnacle growth which always occurred on ships' hulls. After a two-year...

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What is galvanic corrosion? Galvanic corrosion or "Bimetallic Corrosion" or "Dissimilar Metal Corrosion", as sometimes called, is defined as the accelerated corrosion of a metal because of an electrical contact (including physical contact) with a more noble metal or nonmetallic conductor (the cathode) in a corrosive electrolyte.

The less corrosion resistant or the "active" member of the couple experiences accelerated corrosion while the more corrosion resistant or the "noble" member of the couple experiences reduced corrosion due to the "cathodic protection" effect.

The most severe attack occurs at the joint between the two dissimilar metals. Further away from the bi-metallic joint, the degree of accelerated attack is reduced.

In this photo, a 5-mm thick aluminum alloy plate is physically (and hence, electrically) connected to a 10-mm thick mild steel structural support. Galvanic corrosion occurred on the...

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Heat removal is one of the most important functions of a metalworking fluid. Effective heat removal yields good tool life and dimensional accuracy of parts. Water has greater capacity for removing heat than oil; however, water alone in contact with freshly machined metal leads to corrosion. Thus, corrosion is a problem faced by every user—and manufacturer—of water diluted metalworking fluids. However, corrosion can occur even with dry cutting and is not simply due to the use of water-based fluids.

Seasonal Corrosion

Corrosion can occur at any time during the year, but normally it occurs more often during July, August and September when temperatures and the relative humidity are high. This applies to the Eastern and Midwestern areas of the United States. Corrosion is not usually as much of a problem in arid states, such as Colorado, New Mexico, Arizona, Utah and California, because the relative humidity is usually low.

As temperature increases, the rate of all...

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