Antifriction and anticorrosive phosphating

 

Introduction

 

At present tubes (pipelines, casing tubes, drill pipes ) surface preparation and their junction elements (couplings) preparation for oil industry becomes more and more important aspect for tube manufacturers. Oil industry requires delivery of tubes with repeated usage junctions without jamming.

 

To satisfy tube manufacturers increasing demands MetalChem worked out surface finish processes and chemical materials for these processes.

 

 

1. Junction elements importance, generally accepted processes

 

Oil-producing industry with the aim of expenses saving more often demands threaded joints being split without jamming and again connected and in this way suitable for application on new place of manufacture. These requirements apply both to couplings and tube threads.

 

In industry according to accepted leading guidelines API, the minimal requirement is fourfold connection and disconnection of threaded joints without jamming traces appearance. But this guideline has not already satisfied with modern requirements.

 

In practice there is the demand for possibility of tenfold connection and disconnection of threaded joints. These requirements can be satisfied only by using certain surface finish process. This is especially important for unconventional special threaded joints different from API.

 

During several years four traditional methods of surface finish have been formed:

- Zink-phosphating

- Manganese-phosphating

- Galvanic zincing

- Bead-blasting processing

 

MetalChem, first of all, develops and produces chemical materials for surface finish, that's why last two methods mentioned above we'll describe briefly.

 

Galvanic zincing forms plastic layer on the surface with about 10 µm thickness. The layer satisfies declared requirements and considerably exceeds them in anticorrosive protection properties. But the major disadvantage is high expenses both capital investments and production costs which are much higher than phosphating costs.

 

Bead-blasting processing applies only for thread processing of high-alloy, chrome-bearing and rather soft steel which surface cannot be processed using chemical methods because of alloying additions presence. After bead-blasting processing with special small-grained material application the surface of thread becomes firm that provides detachment and junction without jamming.

 

In practice the most widespread decision is phosphating during which on the thread surface, depending on steel grade and tube type, owing to chemical reaction crystalline zinc- or manganese-phosphate layer is being generated.

 

Method popularity is justified first of all by less cost in comparison with galvanic zincing.

Phosphatic layer has a multifunctional purpose, ensures oil industry requirements:

 

- Jamming protection;

- Anticorrosion protection;

- Sealing improvement.

 

As mentioned above, antifriction properties of phosphatic layer do not excel  these properties in galvanic zincing method but correspond to practical market requirements.

 

This is especially true concerning manganese-phosphate layer which hardness and antifriction properties are much better in comparison with zinc-phosphate layer. The degree of friction decreasing can be improved by further layer impregnation by certain oil.

Despite the density of manganese- and zinc-phosphatic surfaces they contain a multitude of pores which can be filled with emulsion that at the same time increases endurance properties and corrosion stability.

 

The best decision is use of the emulsion of oil that was developed specially for this purpose and contains a number of special antifriction and anticorrosive additions.

 

In some cases final oiling is not used for temporary protection of the phosphatic surface from corrosion especially on the tube endings thread but an organic surface based on amino alcohol is applied.

 

In these cases to provide thread compaction it is applied the special lubricating material (API 5A3) which provides protection of the phosphatic layer. Gas impermeability of tube threaded joints with thread compaction lubricant application increases in the presence of phosphatic surface on thread because porosity of the phosphatic layer improves lubricant adhesion.

 

In this way phosphating also increases production efficiency that becomes more and more important aspect in oil industry.

 

 

Zinc-phosphating

 

During zinc-phosphating the hopeit (Zn₃(PO₄)₂ . 4H₂O) layer forms on the surface but the layer always contains also the iron-containing zinc-phosphate, phosphophilit(Zn_2­Fe(PO­­₄)₂ . 4H₂O).

 

There are following chemical reactions during phosphating process:

 

Etching reaction: Fe⁰ + 2H⁺ → Fe²⁺ + 2[H⁰]

 

Depolarization: 2[H⁰]+ [O] → H₂O

 

Layer forming: 3Zn²⁺ + 6H₂PO₄ + 4H₂O → Zn₃(PO₄)₂ x 4H₂O (HOPEIT)+ 4H₃PO₄

 

2Zn²⁺ + Fe²⁺ + 6H₂PO₄ + 4H₂O → Zn₂Fe(PO₄)₂ x4H₂O (PHOSPHOPHILIT) + 4H₃PO₄

 

Slime forming: 2Fe²⁺ + 4H₂PO₄ + [O] + H₂O → 2FePO₄ x 2H₂O + 2H₃PO₄

 

In depolarization and slime forming reactions the oxygen is an accelerant for phosphating baths. The accelerant is always a strong oxidant in cases when phosphating concentrate contains so-called inner accelerant.

 

Applied type of the accelerant is defined first of all by thickness, mass and structure of the zinc-phosphate layer. The volume of this article doesn't make it possible to carry out the detailed characteristic of certain phosphating processes that's why only the most corresponding to threaded joints processing phosphate types will be examined.

 

On the basis of requirements for mechanical properties and anticorrosive protection of the thread the most optimal surfaces are surfaces with high weight and rather high hopeit content.

 

These two factors have contradictions because the surface with high specific mass can be reached by so-called iron-sided phosphating (i.e. with addition agent) without the additional accelerant.

 

The main characteristic of this process is the fact that the accelerant is the nitrate that is contained in phosphating solution itself and its concentration is much higher in phosphating compositions specially developed for these purposes than in other cases when another accelerant (nitrite, chlorate etc.) is also used. Nitrate ions oxidize hydrogen ions in depolarization reactions generating water but are not strong enough oxidants to oxidize Fe(II) ions in Fe(III) ions in etching reactions.

 

In that case phosphate slime (iron phosphate(III)) does not be generated and concentration of dissolved iron(II) increases during phosphating process. Owing to content of dissolved iron(II) in solution the risk of forming the phosphate layer, where phosphophilit becomes prevalent, increases that leads to mechanical properties deterioration. To solve this problem we raised greatly concentration of zinc in phosphating material that was developed for this purpose, providing appropriate high percentage of hopeit. During phosphating process nitrite can be generated from nitrate in side reactions and it can hinder phosphating process:

 NO₃^- + 2H⁰ → NO₂^- + H₂O

 NO₂^- + 3H⁰ + H⁺ → 1/2 N₂ + 2H₂O

These side reactions hinder the operation of working solution and process controllability, that's why special additive for decomposition of the generated nitrite is used.

 

On the base of the above we developed three-component phosphating concentrate:

 

- METAPHOS ZN 510.2 Prep for preparation of the phosphating solution;

- METAPHOS ZN 510.2 Alim for correction of the phosphating solution, concentration support;

- METASTARTER N - additive for decomposition of nitrite.

 

 Above-mentioned materials are completely ready for use.

 

Both of the materials contain appropriate components in balanced proportions and provide good controllability, operation stability of phosphating surface and phosphate surface forming.

 

Operational parameters of the phosphating bath:

 

Concentration:

140 g/l METAPHOS ZN 510.2 Prep.

1 g/l METASTARTER N

 

Total acidity: 70±5 dots

Iron(II) content: 4±1 g/l

Temperature: 80±5^oС

Processing time: 3 - 15 minutes

 

However, these parameters are only approximate. According to conditions of equipment operation and brand of processing steel our specialists can offer parameters that differ from the above.

Phosphating process is not the one-step process, it consists of preliminary and consecutive operations which are as important as the phosphating stage.

 

In the table №1 there are the main operations of the phosphating process:

 

Table 1

№	Process	Product	Temperature оС
1	Degreasing	METADIP series	60-80
2	Rinse	Process water	Surrounding
3	Etching	Sulphuric and hydrochloric acid	Depends on a process
4	Rinse	Process water	Surrounding
5	Rinse	Process water	Surrounding
6	Activation process	METACOND series	Surrounding
7	Zinc-phosphating	METAPHOS series	80
8	Rinse	Process water	Surrounding
9	Rinse	Process water	Surrounding
10	Oiling and passivation	PROTECT series	70

 

Degreasing is the most important operation, because the surface of appropriate quality forms only on the clean, degreased surface of the metal. During the threading process the lubricoolant emulsions, which dry out subsequently on the surface, are used. Mechanical contaminants (scale powder, dust, etc.) often adhere to this oil layer.

 

These contaminants can be removed by strongly alkaline degreasers which along with inorganic matters contain significant amount of surface-active materials (SAM).

 

Type of applicable SAM depends on degreasing method, but because degreasing usually carries out by embedding method, along with nonionic (ethoxylated fatty alcohols) SAM we almost always use amino compound anionic SAM to provide appropriate oil scumming.

 

Some of our requesters apply spray phosphating method for muff and tube threaded joint phosphating but in this case the anionic SAM application is not required.

 

One or two rinse operations follow each chemical processing (with the exception of activation)  to remove the remains of chemical materials from the surface. It provides efficiency of subsequent chemical operations. Process and tap water with the certain requirements for the quality is usually used for rinse.

 

Acid etching is only requiredwhen the phosphating surface is rusty. Muffs are manufactured on metal-cutting machine tool, that is why there is no scale on their inner surface. The outer surfaceis usually covered with scale but this surface is not destined to phosphating, that is why it is not necessary to remove oxides from it.

 

In practice etching is used very seldom, because capital investments in equipment and environmental protection expenses are very high. It is much easier to prevent rust formation on the machining surface (e.g. using quality lubricoolant)  then to introduct acid etching operation.

 

Activation process is of greatest importance to provide appropriate crystalline phosphate surface formation. In practice activation process represents itself as the rinse operation with application of the chemical material solution immediately followed with the phosphating operation.

 

Activating compositions contain components which lead to so-called crystallization centers formation on the surface. It provides simultaneously uniform start of the phosphate crystals formation that leads to multitude of small crystals formation which completely cover the surface. Porosity of such covering provides absorption of huge amount of the oil.

 

Phosphating and its major factors was examined above in details.

 

Phosphated and rinsed surface has not provided essential anticorrosive protection yet. That is why it is necessary to perform appropriate subsequent processing for pore compaction of the phosphate surface.

 

Following processing depends on the thread type and mark of the steel, requirements of the customer, traditions of the plant, but it is usually oiling or passivation  by the organic product

 

For this purpose MetalChem recommends the special anticorrosive oil (PROTECT 1882) that is used as 58% water emulsion which is applied on the surface providing appropriate pore compaction.

 

If the use of oil-containing emulsions is not desirable we recommend to apply 1-2% water solution of the passivator that contains amino alcohol.

 

Manganese-phosphating

 

The mechanism of the manganese-phosphating process is similar to zinc-phosphating process, the surface consists of the ferric manganese-phosphate ( (MnFe)₅H₂(PO₄)₄ x 4H₂O ).

 

Mechanical properties of the manganese-phosphate layer is much better in comparison with zinc-phosphate layer, therefore this layer is recommended for the phosphating of such threaded joints that are subjected to heavy load and that should provide repeated connection and disconnection.

 

The specific mass of the manganese-phosphate surface is also rather high (it often reaches 15-20 g/m²), that corresponds to 8-12 µm thickness of the surface. The ability to absorb oil is also much higher in comparison with zinc-phosphate.

 

The manganese-phosphating product METAPHOS MN.502 provides the best results of the processing of the threaded joints.

 

The operational factors of the solution:

 

- Concentration: 160 g/l METAPHOS MN.502

- Total acidity: 60±5

- Free acidity: 11±1

- Dissolved iron(II): 2-3 g/l

- Temperature: 95 - 98^oC

- Processing time: 10-15 minutes

 

Technological process of the manganese-phosphating has two essential distinctive features in comparison with the zinc-phosphating.

1) Higher phosphating temperature, 95 - 98^oC. Secondly, phosphating process requires in addition to high temperature its holding in such a short range. Long-term solution usage at the higher or lower temperatures can disturb balance of the process solution and eventually lead to loss of its phosphating ability.

 

2) Another distinctive feature is the fact that, despite the outer accelerant is applied in this process, the dissolved iron concentration increase does not take place until the certain equilibrium level (as it does in the zinc-phosphating process description) but the forming nitrite ions oxidize the iron(II) to iron(III) which generate with the phosphate ions of the bath  insoluble iron(III) phosphate in the form of the slime.:

 

The slime generation in the manganese-phosphating bath is very essential fact, that should be taken into account when developing phosphating equipment and estimating expected expenses.

 

Stages of the manganese-phosphating process completely correspond to the operations for the zinc-phosphating.

 

Significant difference is only in the structure of the activating chemical material.

 

We recommend to apply the two-component activating product METACOND 20A, that contains water-insoluble special manganese salt, grinded to the colloidal size, and METACOND 20C that is the buffering component, first of all needed for maintenance of the pH value and for the colloidal blend stabilization.

 

Thus the activation performs with the suspension assistance, that should be uninterruptedly blended to provide appropriate efficiency. This method of the activation provides permanent and fine quality structure of the manganese-phosphate.

 

Manganese- or zinc-phosphating?

 

The question is: What is better to choose: manganese-phosphating or zinc-phosphating?

 

The simplified comparison of these two processes are showed in the table 2:

Table 2

Characteristic	Manganese-phosphate	Zinc-phosphate
Corrosion protection	higher	lower
Endurance	better	worse
Chemicals expenses	higher	lower
Energy intensity	higher	lower
Slime generating, environmental protection expenses	higher	lower

 

Though the manganese-phosphating is the process of higher level, it is more expensive process at the same time. A choice of the process depends on customer's requirements and sometimes marks of the steel.

 

For further information or technical offer on the above-mentioned materials and processes contact the Technical Service.

 

MetalChem company provides its customers technical support during developing of the technologies, tests and also ensures regular technical service during the whole period of applying its materials.

 

Reprinting or copying only in agreement to MetalChem LTD.