Neph Sy solubility and aluminum

[Bryan Johnson]

I am looking for some more details on aluminum pitting and the hard material that forms in proximity to the aluminum.

The soluble sodium must be reacting to something, but what is formed when it dissolves? Does that play a role/or is the main contributor?

 

 

[neilestrick]

Could you be more specific? Are you talking about clay or glazes? Do you mean aluminum pitting in a pug mill? 

[Bryan Johnson]

I am mostly interested in what happens with Neph Sy in clay.

The explanations that I have read seem incomplete. Neph Sy is often said to have soluble sodium- where is that from and what form does it take?

 

[liambesaw]

Yes, it is analagous to a soda Feldspar.  If you are interested in a "general" composition you can visit here: https://digitalfire.com/4sight/material/nepheline_syenite_1069.html

Under normal circumstances, nephyline syenite would remain insoluble, because the soluble elements are encased in stone and crystalline matrix (like a glaze).  However when ground into a fine powder some of these bonds are broken and it can become slightly soluble.  This is how it can affect some glazes and clay bodies (if used as a Feldspar in a clay body).  

[Bryan Johnson]

 I think that what I am trying to figure out is why the nepheline, the component that is missing a silica would react with the oxide layer or the metallic aluminum to form pits and a hard deposit. Nepheline should react with silica to form feldspar, not with alumina.

 

[liambesaw]

28 minutes ago, Bryan Johnson said:

 I think that what I am trying to figure out is why the nepheline, the component that is missing a silica would react with the oxide layer or the metallic aluminum to form pits and a hard deposit. Nepheline should react with silica to form feldspar, not with alumina.

 

Because the soluble salts in neph sy facilitate the movement of ions.  This doesn't cause oxidation itself but it allows oxidation to occur more rapidly. Also the silica and alumina in clays are in oxide form already, and the aluminum metal in the body of the extruder is not.

[Bryan Johnson]

The critique of neph sy is that it breaks down the oxide layer(reduction?) and also the aluminum (oxidation?).  Years ago Ivor Lewis, on clay art said that the oxide layer is amphoteric, so I suppose it could also be oxidation . 

Then to counteract the problem we add epsom salts, a mild acid, which seems like should be more likely to break down an oxide layer.

[Magnolia Mud Research]

High pH from sodium solvated from Nephsy will dissolve the oxide layer of aluminum.  (Sodium hydroxide is the solvent used in processes converting bauxite to aluminum).  Adding a reagent that will lower the pH will stop the corrosion; if the pH goes too low acid corrosion begins again .  Some where between too much and too little pH will be the optional place to operate.   Or change the metal to a grade that fits the natural pH of the material being used in the equipment.  

[Bill Kielb]

11 hours ago, Bryan Johnson said:

The critique of neph sy is that it breaks down the oxide layer(reduction?) and also the aluminum (oxidation?).  Years ago Ivor Lewis, on clay art said that the oxide layer is amphoteric, so I suppose it could also be oxidation . 

Then to counteract the problem we add epsom salts, a mild acid, which seems like should be more likely to break down an oxide layer.

Not sure if we talking about pug mills or corrosion in general but in general, all corrosion is ionic and soaps or higher ph mixtures facilitate erosion readily. Dishwashers in general are real tough on glazes for sure.  They dissolve them away slowly by removing and exchanging smaller ions leaving a less dense structure. If in a pug mill Any mechanical abrasion will remove the freshest layer of oxide and any mixture that promotes corrosion in metals will do just that.

A good somewhat related  read with respect to erosion  would be the development of lotus or gorilla glass and the ion exchange that provides greater compression by removing sodium and replacing with potassium. A compression  and pre ion exchange approach to strength and longevity. 

Another interesting moderately related read would be cathodic protection and corrosion in metals. In general  on a larger scale we simply run a small electric current in reverse so to speak to provide a barrier against what is basically a movement of electrons issue. Ph is significant in promoting this movement as @Magnolia Mud Research mentioned above.

[Bryan Johnson]

Thanks for the insights.