Dezincification?

[ArthurWood]

I recently noticed that the head, a/c and pt engine bronze thru hull stuff had turned a darker exterior color and yesterday when I looked inside the a/c strainer the metal at the bottom was pinkish. Is this dezincification? If so, where should I look first for a problem? Transom zinc looks OK.

 

[steve_cronin]

Pink = dezinc, Arthur

But as to the "darker colo(u)r. well that is a mistery to me.

De-zincification is the leeching out of the zinc from the brass alloy leaving the random matrix of the other component, copper, behind. This pure copper appears pink in contrast to the golden colour of the remaining brass. It of course severely weakens any fittings and means their immediate replacement. It is very common in "P" brackets (or Prop struts in USAian) and skin fittings. The latter can cause the engine to be unusable as failure will result in the propshaft not being restrained at the outboard end. In skin fittings it can result in sudden failure of the fitting particularly under the backnut where strain is greatest. I have experience of such a failure and I can tell you that the resultant panic rush from three miles offshore to beach with a towel stuffed in the hole was an experience I never intend to repeat!

If you have a/c on your boat you must have quite a lot of holes in the hull. Ever thought of the single stack system?

Enjoy sunny Fla. How many "F's" there at the moment?

Regards

Steve CRonin

Above edited as of course following Vyv's posting I realised that you were talking about bronze not brass. If your fittings are covered in pink blotches they are brass and you have problems!

<P ID="edit"><FONT SIZE=-1>Edited by steve_cronin on Fri Feb 1 15:57:06 2002 (server time).</FONT></P>

 

[vyv_cox]

Don't understand your abbreviations, so no detailed comment. Bronze has no zinc in it, so if there is dezincification it is brass. Bronze always goes darker but not redder. Green surface deposits are not unusual. The strainer gauze inside a water intake is often brass, so this is no cause for concern.

 

[AndrewB]

Confusion of terminology?

Bronze, like brass is a copper alloy. Should it be subjected to electrolysis, it may go copper-coloured (pinkish) as other metals are leached, specially in poor grade alloys.

But there are many types. The usual bronzes found in the UK marine industry, copper-tin alloys and phosphor bronze, are fairly cathodic and unlikely to be attacked by most other metals normally used aboard (very slowly if stainless steel bolts are used). In the US, bronze includes copper-aluminium alloys and alloys with a proportion of zinc (manganese bronze), generally classed as a brass in the UK. These are attacked and do go pink. But I would be suprised to find them used on a marine engine.

There is an interesting site at http://marinesurvey.com/yacht/corrosion_1.htm.

 

[lezgar]

Dezincification (http://corrosion-doctors.org/Forms/dezinc.htm)

Dezincification selectively removes zinc from the alloy, leaving behind a porous, copper-rich structure that has little mechanical strength. An in-service valve suffering from dezincification has a white powdery substance or mineral stains on its exterior surface. The valve may exhibit water weeping from the valve body or stem/bonnet seal.

What's the cure? A tightly written valve specification that limits brass alloys to those containing no more than 15% zinc, or specification of proven dezincification-resistant yellow brass alloys, say the experts. Further, manufacturers must be required to provide alloy designations or chemistry for the materials used in their valves and fittings. Over the past decade, an evolution in alloys has occurred, and yellow brasses that are dezincification-resistant do exist. However, specifiers who simply accept inexpensive yellow brasses without regard to whether they are standard alloys-or even meet the performance requirements of standard alloys-are vulnerable to potential dezincification problems.

Why Dezincification Occurs Copper-zinc alloys containing more than 15% zinc are susceptible to dezincification. Zinc is a highly reactive metal, as seen in its galvanic series ranking. This reactivity stems from the fact that zinc has a very weak atomic bond relative to other metals. Simply, zinc atoms are easily given up to solutions with certain aggressive characteristics. During dezincification, the more active zinc is selectively removed from the brass, leaving behind a weak deposit of the porous, more noble copper-rich metal.

Two types of corrosive attack characterize dezincification: plug and uniform (or layer). Plug-type dezincification is localized within surrounding surfaces mostly unaffected by corrosion. This type of dezincification penetrates deeply into the sidewalls of valves and fittings. Common failures associated with plug-type attack include penetration through the sidewalls that causes water seepage or loss of mechanical strength in threaded sections to the point of fracture. Uniform-layer dezincification leaches zinc from a broad area of the surface. This type of dezincification uniformly reduces the wall thickness of the valve or fitting. A complex set of conditions must be present for dezincification to occur, and the occurrence is often related to region of the country.

The service conditions generally present where dezincification occurs include: Water with high levels of oxygen and carbon dioxide (uniform attack).

Stagnant or slow moving waters (uniform attack).

Slightly acidic water, low in salt content and at room temperature (uniform attack).

Soft, low pH and low mineral water combined with oxygen, which forms zinc oxide (uniform attack).

Waters with high chloride ion content (uniform attack).

Neutral or alkaline waters, high in salt content and at or above room temperature (plug-type attack).



Common signs that dezincification is occurring include: Presence of a loosely adhering white deposit of zinc oxide on the exterior of the valve.

Presence of mineral stains on the outer surface of the valve.

Water weeping from the valve body or stem/bonnet seal.

 

[ArthurWood]

Thanks for the treatise. The components don't show any of the symptoms described and I'm not surprised because they are bronze and should not contain zinc. I should have known better (BSc Chem), but I had read somewhere about hull fittings going pink and thought it was dezincification. It's a bit puzzling, al the same.

 

[ArthurWood]

Re: Pink = dezinc, Arthur

Hi Steve, Well, the seacocks are bronze as is the strainer assy, so I don't understand the pink colour. No blotches, just even pink inside the strainer. Also noticed it inside genset water pump. Yep, have two a/c units but what's a stacked system?
Whisper: 5.45pm, sun low and 75 "Fs"

 

[lezgar]

About Bronze

Hope this help

Bronze (http://corrosion-doctors.org)

There are a limited number of major chemical changes that happen to bronze as it deteriorates, depending on age, soil conditions and a few other things that are collectively referred to as 'the conditions of preservation.' The visible results of the changes in the bronze are collectively referred to as the patination, patina, encrustation, or verdigris. Pretty much all of the changes that occur in bronze over time are the result of interactions of the copper in the alloy with the environment. Tin is relatively inert and is stable in alloy with copper.

Pretty Patina


Copper Oxide, Cupric Oxide, Cuprous Oxide: (red, brown or black). Cuprous oxide is generally reddish in color and tends to form first. It quickly converts to cupric oxide which is dark brown or black in color. Virtually all ancient bronze coins have at least a thin layer of brown copper oxide directly on the metal surface.

Copper Sulfate or Sulfide (green to black)

Copper Carbonate (accounts for most green patina and occasional blue). Copper carbonate is a reaction to copper oxide, not to copper, so it will only form on top of the brown or red copper oxides. Because copper oxide is more stable than copper carbonate, the green can sometimes be selectively removed leaving the red or brown.

Copper Acetate (green, occurs frequently with copper carbonate)

Destructive Patina


Cuprous Chloride and Cupric Chloride (pale green powdery spots on the surface of a coin or artifact). While there are generally a number of reactions occurring on the surface of a coin at any given time, the presence of chloride ions is the most destructive, since they produce hydrochloric acids which eat your coin thus producing more Cuprous Chlorides to... etc. until there is no artifact.

Reddish Warts - we are not sure what this is, but its bad. This is scaly bumps usually 3-7mm high that form on the metal. It can be removed but is extremely destructive, leaving large pits and destroying most details. Bronze that shows this type of degradation is frequently unstable and soft even in the areas not directly affected by the warty encrustations themselves. I personally will not buy pieces with this symptom as they tend to look bad and do not respond well to cleaning.

 

[longjohnsilver]

Gales

Well Arthur you can whisper all you like, we won't hear you over the howling gales over here. Taunton town centre closed yesterday cos of falling tiles, trees down, people (silly people!) swept into v rough seas.

You just relax in your a/c comfort, soon be the hurricane season!!

PS off to a wedding in Wellington today, reception at Rockwell Green village hall!!!! Can't wait!!

<P ID="edit"><FONT SIZE=-1>Edited by longjohnsilver on Sat Feb 2 09:20:14 2002 (server time).</FONT></P>